You CAN flash a 1.2 xbox...
Here is a link about the whole thing. You have to have a 256k bios though. You can download tools that will turn a 1MB bios into a 256k bios.
It will work in Windows XP, and I myself have done it more than once. All you have to do once you have the save running is go to start and click on run. Once in run, type in "telnet 192.168.0.64" without the quotes (you must be on a 192.168.0.* gateway), username is "root" and the password is "xbox" again, both without the quotes.
Yes, it works with the Greatest Hits version.