multiple OS question

Ok so here's what my friend did with his computer.

He got it with XP home. The drive came partitioned into C and D. CD drives as E and F.

Later he added a bigger hard drive that went on as G (not sure why it didn't go on as E and displace the cd drives, but that's neither here nor there)

He wanted to resintall windows cause it was all funked up after a few years, but lost his disks and such. So he got XP Pro on the cheap from school.

He decided to leave the main drive alone and install XP Pro on the G drive, so it loads with a boot menu now to choose between the two OSs. I think the reasoning was because he doesn't remember where all his software comes from, so he could boot to the C in case he needed something. But the eventual plan was to clear off the C once he was fully migrated.

Something about this seems like a bad idea. If he later formats C, will the computer know to boot from G? I'm pretty sure that drive will be stuck as G unless he reinstalls windows again right?
Are there any other reasons why having G as your windows drive could be an issue? I know my dad did an install once where it named his drive F instead of C, and it worked except one program wasn't happy with it.


sidenote: My friend used the motherboard I had when I tried to build a computer, and it worked fine for him, so I guess that wasn't my issue.