I just reinstalled XP Pro and now my main drive with Windows XP on is drive E and my second hard drive is drive C (with no operating system on it). What has happened?? How can I get my main HD to be C: ???

Any help welcome.

Something happened with the way XP lettered the drives.  I've seen this happen with systems with internal ZIP drives.

Bad thing is that you can't change this after the fact.  There are too many registry settings affeted.  Either live with it or disconnect your old drive and reinstall XP.