Mine just did this too. If you look in C-> Documents and Settings There will be one folder named temp that it is using now and your original username on another folder. You files are there. I am working on a fix for mine. I am sure there is a way to restore everything I just don't know it now.
Either the account folder got deleted or your profile is corrupt. In either case, log in as admin, you may have to from safe mode. Create a new profile either through user accounts in control panel or computer management. Then just copy your My Documents and Favorites from the old account's folder to your new one.
So long as you installed everything with access for any user that you had a choice to, you shouldn't have to reinstall much if anything. Run a chkdsk as well. May not be an issue because this can happen without any logical reason, but it's not bad to check.