By Dennis O'Reilly Reverting a Vista PC to XP requires an installation CD for each OS and can be done only on OEM editions of Vista Business and Ultimate. Users of Vista Home Basic and Home Premium — and anyone who used a retail version of Vista to upgrade an XP machine — must buy a copy of XP to make the switch.

Last week's Top Story on Microsoft's decision to extend yet again the deadline for buying a PC with Windows XP installed caused many readers to wonder whether they could dump their copy of Vista in favor of its predecessor. Reader Jim Harvey put it this way:

• "We have Vista Home Edition installed on a newly refurbished Gateway computer purchased for my wife for Christmas. However, trying to cope with all the operational changes in Vista has proven to be too frustrating for her.

  "We would like to downgrade the new computer back to the old XP license we have on our replaced computer, but we don't know how to do so. Is there a legitimate way to install our old licensed version of XP , still on the replaced computer, onto our new Gateway and get rid of Vista?"
  

Unfortunately, the only way you can revert a machine running Vista Home Basic or Home Premium is to buy a copy of XP and install it over the Vista configuration. However, anyone who bought a PC with an OEM edition of Vista Business or Vista Ultimate can downgrade to XP Pro.

Fred Langa's Jan. 8, 2009, column (paid content) described several ways to recover hard-disk space. Reader Kevin Kleinhomer wrote in to remind us of a couple of other tools that might help track down the missing bytes.

• "In his most recent article, Fred talks about a reader with missing space, but I think he missed a very important tip for the reader: Chkdsk. It could be a corrupted file system that is the root cause of the missing disk space. I have seen this many, many times.

  "A less likely possibility would be a rootkit. Booting off one of the many recently reported-on [rootkit-revealing] tools would hopefully turn this up."
  

Running Windows' built-in disk-checking utility couldn't be easier: click Start, Run (in XP) or just Start (in Vista), type cmd, and press Enter. At the command prompt, type the following:

The rap on Linux — at least among Windows users — has long been that the alternative OS is too difficult to install and use. Scott Spanbauer's Jan. 8, 2009, Best Software column (paid content) described the free Wubi installer utility for the Ubuntu distribution of Linux. Reader Howard Harner points out that you can also get a free copy of Ubuntu on disc, if you're patient.

• "I'm glad to see your discussion of Ubuntu, since I have been using it as an alternative to uSoft [Microsoft Windows] for years. For older computers, cruising the Web, and copying CDs, it's great.

  "You didn't mention that one can get a free disk from Ubuntu that contains two versions of the OS — a full-install copy and a version that will run on top of Windows — by going to their Web site and filling out the short application form. It usually takes less than two weeks to receive it."
  

In fact, many Windows users choose to run Ubuntu off the CD rather than to create a hard-drive partition for the OS. Of course, you can burn your own Ubuntu CD. You'll find the download and instructions for creating your disc on the Ubuntu Community Documentation page.