I am considering the purchase of the MacBook Air (AAPL). If this computer freezes up, as my current Dell Inspiron does on occasion, how would I correct for that? I usually remove and then replace the battery on the Dell (DELL). But what would I do if this happens with the Air, since it has no removable battery?

You can force a frozen MacBook Air — or any current Mac laptop — to shut down and restart without removing its battery. You just hold down the power button for a few seconds until the machine shuts off, then wait a few seconds, then turn it on again.

I am a Verizon (VZ) customer and have a Palm (PALM) Treo 700W for personal use. I would like to replace the Treo but, in light of Verizon’s announced “open network” policy, I am wondering if this is the right time.

It’s far too early to know what new phones will be available under the new Verizon program, since that depends on which phone makers decide to make devices to run on the “open” network, and what kinds of devices they might produce. There is the potential for phones that are more creative or interesting than what is available today, but it’s only a potential. Verizon has just laid out the first details of the plan, and no product announcements are likely for quite a while. If your Treo needs replacing soon, and you want to remain a Verizon customer, you will have to choose from the company’s current smart-phone offerings.

Are there any cellphones that allow for two separate phone numbers? Right now I carry two devices, both with the same carrier.

At least in the U.S., each cellphone, or each SIM card that goes in a phone, is normally tied to a single number. To use two numbers on a single phone, you could purchase an unlocked phone running on a GSM network and then purchase two SIM cards, each with a separate number, and swap them in and out. But that would be an inconvenient process, since inserting and removing SIM cards typically requires opening the phone and even removing the battery.

You can find Mossberg’s Mailbox, and my other columns, online free of charge at the new All Things Digital Web site, http://walt.allthingsd.com.

There’s no other major item most of us own that is as confusing, unpredictable and unreliable as our personal computers. Everybody has questions about them, and we aim to help.

Here are a few questions about computers I’ve received recently from people like you, and my answers. I have edited and restated the questions a bit, for readability.

Last week you discussed how to uninstall Windows Vista and go back to Windows XP. But I own a Macintosh, and after upgrading to the new Leopard operating system from Tiger, I find I dislike Leopard. How can I uninstall Leopard and go back to Tiger?

To restore the older Tiger operating system after upgrading to Leopard, you should first find the Tiger DVD that came with your Mac. Insert it and reboot your Mac while holding down the “C” key. This will boot the computer from the DVD rather than your hard disk.

On the screen that shows available hard drives for installation, click on the Options button and select the “Archive & Install” option. Also, select the choice called “Preserve Users & Network Settings.” If you have enough free disk space (roughly six gigabytes), Tiger will be reinstalled and your home directory and applications should be preserved.

However, just as with Windows, performing such an operating system “downgrade” on a Mac can be tricky for a nontechnical user. You might want to hire an expert to do it. If you do decide to try it yourself, I strongly urge you to first read an Apple document that contains more details, including some potential pitfalls and limitations of the “Archive and Install” procedure. It’s at http://docs.info.apple.com/article.html?artnum=107120.

Occasionally, I try to download a song or artist that would be in the “Easy Listening” category, but neither iTunes nor Amazon lists that genre. How can I find them?

In the iTunes store, there is indeed a genre called “Easy Listening” that contains thousands of tracks. The problem is that it’s not listed on the front page of the store. To find “Easy Listening” and the songs it contains, select “Browse” from the box labeled “Quick Links” at the upper right on the store’s front page. Then click “Music” in the far left column and “Easy Listening” in the column labeled “Genre.”

In Amazon’s MP3 download service, I couldn’t find a listing for the “Easy Listening” genre. But some of the songs and artists you’re looking for could be listed under other categories. Try directly searching for an artist’s name or a song title in the search box at the top of the MP3 Downloads page.

Why isn’t a FireWire port included on either of the two new slim laptops you recently reviewed, the Lenovo ThinkPad X300 and the Apple MacBook Air?

Both laptops are so small that they can only include a limited number of ports, though the Lenovo has many more than the Apple. And standard FireWire, also known as “1394″ or “iLink” on some machines, is becoming redundant in mainstream consumer computers, since the USB 2.0 ports offer roughly the same speed and are compatible with many more devices. There is a faster version of FireWire, which Apple uses on most of its desktop models. But there is also a faster version of USB in the works.

You can find Mossberg’s Mailbox, and my other columns, online free of charge at the new All Things Digital Web site, http://walt.allthingsd.com.

Corrections and Amplifications

Due to incorrect information provided by the manufacturer, this column erroneously says that Macintosh users performing a “downgrade” from Apple’s new Leopard operating system to the older Tiger system should select an option called “Preserve Users & Network Settings.” In fact, that option isn’t available when installing an older version of the operating system over a newer one. Users must instead manually relocate their data files and settings after performing the downgrade, a process that is explained at this Web page: http://docs.info.apple.com/article.html?artnum=107297.

We need a new DVD player but are completely confused about the Blu-Ray versus HD DVD war. We want to purchase something that will last us many years, but I’m afraid to splurge on it when it’s still unclear what the preferred format will be. One option looks to be buying a DVD player that can “upscale” to high definition. How much worse is this in terms of quality?

The good news is that the war appears to be winding down. Because I never saw any significant quality difference between the two high-definition formats, I never recommended one over the other. But most of the major studios have defected to Blu-Ray, so industry experts believe HD DVD is likely to recede as a movie format, though it may find a market as a data format for computers. However, the companies backing HD DVD haven’t given up, so the battle isn’t formally over.

Meanwhile, I have found that, for everyone but videophiles, “upscaling” DVD players are effective at making most regular DVDs look better on high-definition TV sets. And name-brand models can be found for as little as $45, which is less than Blu-Ray players are likely to commonly cost for quite awhile.

In your review of the new MacBook Air laptop, you said it lacked a wired Internet jack but that Apple plans to sell an adapter. Did you test this? When will it be available? And is there any way to use the MacBook Air with a cellphone network for Internet access?

Apple says the $30 wired Internet adapter will ship when the laptop does. I tested an early unit and found it worked fine. As for working on the cellphone data networks, you can use a cellphone modem from one of the network operators that connects via USB instead of a card slot. Unfortunately, I tested one of these, the Novatel USB727 from Verizon, and found that it was too wide to fit in the cramped space around the MacBook Air’s USB adapter. So you will need an extension cable, making for an ungainly attachment.

There are two other options. If you already own a cellphone data modem meant to fit a PC Card or ExpressCard slot, you can buy an adapter that allows these cards to work via a USB port. I tested such an adapter, a $40 model from a company called Siig that allowed me to use my older Novatel V640 ExpressCard modem with the MacBook Air. It worked well but is relatively large and is attached by a cable, so it is also bulky and ungainly.

The final option would be to pair the MacBook Air with a Bluetooth-equipped cellphone that is able to act as a wireless modem. No cables are required for this, but there is some setup. Ironically, one of the phones that lacks this capability is Apple’s own iPhone.

What happened to the Tablet PC version of Windows? Since Vista has come out, I can’t find a Tablet edition.

Microsoft decided to drop the separate tablet edition it produced for Windows XP and build its capabilities into Vista. Unless you have the least expensive version of Vista, called Home Basic, the full Tablet PC capability should be usable on any tablet PC running Vista. But there’s no longer a specific edition labeled that way.

You can find Mossberg’s Mailbox, and my other columns, online free at the new All Things Digital Web site, http://walt.allthingsd.com.