Check out this link ... it explains clearly how to do so in Itunes (Edit/Preferences/Devices) then delete them.
However, I had no backups listed, so down further on the page, there is an explanation of how to delete within Windows 7 ...
"I fixed it by opening the backup folder (:\Users\\AppData\Roaming\Apple Computer\MobileSync\Backup). The backup folder contained one folder, which I deleted. Then I went back to iTunes, right clicked my iPhone and chose Back Up. now everything works fine!
I’m using Windows 7, so the path to the backup folder may be different in your case."
I applied this fix and it totally worked for me ... no more corrupt backup file.
Also, I did shut down my firewall (Zone Alarm) just in case. I am running Windows 7 professional.