Check out Microsoft Virtual PC 2007. (a Microsoft freebie). You can run multiple vitrtual OS’s under this including W2K, XP, W2k Server, W2K3 Server, Vista, various flavors of Linix, etc. Only limited by amount of memory and disk on your physical cpu. I use this product alot. Two thums up on this one.
I used that a lot too, and liked it, until I started using VMWare. Then I realized this is one of those cases where you get what you pay for. VMWare lets the guest have access to more than one processor/core, lets you run applications on the guest without the rest of the OS screen showing (looks like I have a Windows app running in OS X), emulates DirectX (8.1, but expect it to improve), can support 64-bit guests, and can give up to 8 GB of RAM to the guest (VPC max is 3.6).
It is certified to run all Windows versions down to 3.1, DOS, all major Linux distributions, Solaris and BSD. Virtual PC only officially supports Windows 98 and later, and OS/2. You can run others and Linux, but don't ask for help if it doesn't work. You can't even try to virtualize Home Basic or Premium because VirtualPC checks for that.
And it was only $60 ($190 for the Windows version). I'd suggest VMWare if you're doing anything more than playing around.
From what I've read in reviews, Microsoft's Virtual PC doesn't even come close to VMware in terms of functionality.