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You can attempt it.....but going from an Intel solution to an AMD is tough without a reformat.........Greg >>
I've had a lot of succes before without a reformat; in truth, it depends on the OS. If your friend is running XP or 2000, you'll probably be OK. If it's older, esp. Win95, you could be in trouble. Frankly, it can't hurt to try it without reformatting; just back up the data first, and the worst thing that can happen is you have to reformat and lose some time. EDIT--play with Windows for a while before you decide it's working; sometimes minor instabilities take a few hours to show, even if it booted fine.
best way to back up data is to plug in one of your extra HDDs into his old computer before you change anything (hopefully you have extra power+IDE connector, if not, unplug from a CD drive and use that). Then they should both appear under My Computer; just copy everything from the original to the new. If the new one isn't formatted, you'll have to do that first.
Once the important stuff is backed up, change out the mobo and processor and have fun
your friend will probably want to upgrade his hard drive at some point for best performance.