Generally, you can delete any/all backups from within iTunes itself. Click on Edit > Preference > Devices and then highlight the backup you wish to delete. Once you've highlighted the backup, a button should appear that's titled 'Delete Backup..." Press that button and your backup should be gone. Then, you can run another...
If that didn't work, here is an address I found that lists where iTunes stores the backup on on Vista computer: \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
Hope this helps. Holler if you need more.
Marilyn