Microsoft have a free download of Virtual PC 2007, which allows you to run virtual machines on your PC. I think it's going to be a standard feature in Windows 7. I have Vista Home Premium, and the installer tells you that it won't work (it says it's only for higher-end versions of Vista), but if you just let it go through the installation, it seems to be OK.
I had an old copy of Win 2k, so I made a guest OS (i.e. an OS running in a virtual machine), and that worked fine. One useful trick to remember is that if the mouse gets stuck in the virtual machine window, press the right hand Alt key to get it back. So next I decided to try installing Linux as guest OS. There is a blog posting which gives some instructions, but unfortunately it didn't work. With Ubuntu 8.04, a dialogue box popped up with an error; with 8.10 and 9.04, I got a stack trace. Fortunately I found another posting with a solution. The key thing seems to be to add noreplace-paravirt and vga=791 to the command line at various stages. The exact way you do this isn't always as described in the posting, at least for 9.04, but otherwise following these instructions seems to work. The only thing that I've not been able to get to work yet is changing the screen resolution to greater than 800x600. There are instructions linked from the second posting, but they didn't work for me.
Why do it? Well, mostly because I can. Occasionally, though, it is useful to run some things under Linux without having to reboot into it.
(Later) After much searching, I found yet another blog post which contains instructions for changing the monitor resolution. This worked!