Microsoft is offering Virtual PC 2004 for free. Grab it at the link above.
For those of you who aren't familiar with VPC04, it's Microsoft's virtualization toolkit for Windows. Reviewers say it's pretty good stuff. I intend getting my copy very soon, as soon as I'm done downloading the 1.1 GIGABYTES of updates to my various systems over dialup!
I think this is one you really need to get, if only to have it around. Support for Vista (as much as that is possible) is included.
Previously, the server version of Microsoft's virtualization framework was also made available as a free download. Grab it here.
I will also remind you that VM Ware is available for free, as well. Get that, too, burn it to CD and tuck it away, if you can resist experimenting.