Windows Back Up Fails

I am running a home built Vista Ultimate machine with 2.5 ghz processor. I have 2 gigs of ram. It is a 32 bit system. I have three drives plus an external Western Digital drive with 500 gigs of storage capacity.

I did a complete system backup back the 6th of September which worked just fine. After that it successfully backed up each week.

I do have a complete system shadow copy but I made the mistake of deleting the previous backups. I thought I would be saving some space (stupid as I have a 500 gig!) but upon running a scan with AVG it showed it couldn't scan the backups.

Now I can't run a backup without it failing. It says I could try doing a new backup but when I do it fails.

Does anyone have any ideas on what I could do to reset this. I could wipe out everything on the external but it is looking for the last successful backup which isn't there.

Please help. Thanks!!!!

I deleted the eternal hard drive and then did a back up. I was able to do the backup and all is well. Thanks for looking. I do appreciate everyone.