Welcome to the discussions,
another thing to try would be deleting the existing backup and try again to backup manually or sync.
But before you try, copy the backup folder to a safe place, in case you'll need it again.
Then delete the backup in iTunes/settings/devices.
Your backup folder is located here:
Windows XP: \Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
copied from:
http://support.apple.com/kb/HT1766