With XP, go to the control panel - admin tools - computer management - disk management and you can fiddle it from there.
Just be aware that any programs that you have installed since the rebuild may have paths to the new and "incorrect" drive letter, but otherwise a nice easy and painless exercise.
Conan
Oh - Another thought. A good idea to sort it out now, as any removable storage will nick the first letter it can get to, so if for instance you put in a card reader, or sometimes plug in an ext drive, the letters can also get a bit jumbled there as well, so specifying a fixed drive letter now, is a good move. Some people put removable drives at the other end, i.e. X,Y,Z but it isn't necessary, if you assign drive letters using the above procedure (I had the same predicament, but with the card reader in, am now up to X anyway!)