As this tends to come up fairly regularly here, particularly when using older software I thought this might be useful.
While it's aimed at Virtualisation beginners, it's not really aimed at PC beginners though...
http://www.reghardware.co.uk/2010/01/19/virtualisation_for_beginners/
These are freeware:
VMware Player can be found at www.vmware.com
Sun's Virtualbox can be found at www.virtualbox.org
This you'll have to pay for:
HyperOS (which isn't really virtualisation, but solves much the same sort of problem) is here www.hyperossystems.com
If you're really a sick puppy, Microsoft's attempt is here, although I think they've locked it down in Windows 7 to running on Professional and Ultimate only. Spend your money on a chocolate teapot instead. No, really.
http://www.microsoft.com/windows/virtual-pc/
Virtualbox is the kind of software I like as it works on just about any platform. It shows the developers have put a lot of effort into it.