Need help changing drive letters in XP

K guys what do i need to do to change the main system disks letter in win xp. I can change all the others no prob but the system disk is set to F: and it will not let me change it. C: is an open letter and the storage drive is set to D:. Its a buddies computer and i dont have the time to figure it out so i need help

I wouldn't recommend changing the system drive letter. A LOT of things depend upon it to be what it is set to during install. Not to mention it is a tiny bit safer as some of the weaker viruses try to install on the C: drive. Changing it would be more trouble than it's worth. If you do, be sure to do a search in the registry and change every entry mentioning F: to C:.
If you absolutely have to change the letter, I would suggest a reinstall.

In that case I would suggest a reinstall too. Windows does not like to have it systems drive letter changed.