Miroku, I'm not a techie, so I won't give you useless info. I too have the Garmin 400 series trainer and find I can run it on either Windows 2000 or XP. If you can find or borrow an old 2000, you should be fine.
I'm not a techie, so I don't buy my own PCs. Every time someone trashes theirs, I take it up. My OSs range from Win 2000, XP to Vista and I am waiting for Win7 to go obsolete. If I can't run a program, I'll feed it to different OSs until one accepts it ... a bit like keeping a few pets at home to keep MS troubles at bay.