Corrupt CAB files

Not sure where this one would fit. So I apologize if it is in the wrong place.have Windows XP SP 2. Lately I have been having a problem with a lot of corrupted CAB files. When I run Avast and it gives me a report page comes up with a list of corrupted files. I try to repair them but i get an error saying there was a problem trying to repair them. The only thing I notice is having slight problems with Firefox and slow shutdowns on puter (etc. programs don't shut down and I have to manually shut them down)
Any suggestions??
Thanks