Since you are able to set up another administrator account, do just that. Lets say you create an adminsitrator account named "testadmin". Log Off, then log back on with testadmin. Since you have administrator privelages, you will now want to re-create the administrator account or refresh it. Go to System Management (right click My Computer and choose Manage), then go to Local Users and groups->users. Examine your primary admin account "Administrator" for anything fishy, and if so fix it. If not, create a new password for that account, and try logging back in to "Administrator" with the new password. If the problem persists and u still have gigs of free space, you may have a hard drive problem, unless you're windows partition ran out of space (which seems highly unlikely here). In the case that you have enough space on all partitions, attempt to run chkdsk or Norton Disk Doctor and Norton WinDoctor and see if that helps any. Note: If you do not have Norton Utilities, or haven't installed them, or even don't wish to buy or use them, you'll have to try chkdsk!
good luck! Let us all know if this helped (I hope it does)

Thanks for your help! The problem was finally fixed by creating a new account and copying all files from the original admin.bak file (in Documents and Settings) into the new account file. (What I hadn't done when I tried this originally was to set Explorer to show all hidden files, so the email settings didn't get transfered.) So now I'm all set.

Excellent! I'm glad you got it fixed. At least I now know about the admin.bak folder, so thanx!