CoLinux is a Windows application similar to VMWare or VirtualPC but it is GPL'ed so there is no cost. Networking is provided by a virtual TAP network driver (installed with CoLinux) which you "bridge" with your existing NIC.
I have been using it all weekend with no crashes...

There are a few downsides: No sound capabilities and USB devices are not supported (other than mouse and keyboard). The CoLinux site states they are working on this though.
More information:
Cooperative Linux: http://www.colinux.org/
Some screen shots of my pc running my virtual desktop
- CoLinux console after boot-up and logon (56K)
- FireFox open on XFCE (145K)
- XFCE filemanager (106K)
- SciTE version 1.61 with a PB example (98K)
- Compiled app using the F5 key in SciTE (97K)
-Beach