agreed, Sun's VirtualBox.com !!!
also, microsoft has Virtual PC 2007; however i do not know if it has Mac binaries.
You can store your virtual drive on either the flash drive or the harddrive, i suggest the harddrive for speed. However, if you want it portable, realize that the software will need installation on each new host you use -- and thus you will have to add settings for your virtual computer on each host also.
Something I have NOT yet tested, is microsoft's Hyper-V server.
http://www.microsoft.com/servers/hyp...r/default.mspx
Apparently it's a standalone product (OS) for virtual computers.
Maybe it can be installed to USB drive, then boot from USB drive. ???