donmac71, Is the ZIP file password protected? Some programs that simply can't open a password protected archive 'think' the archive is corrupted. I'm not sure if Avast behaves that way, but it's possible.
If the cause of this error is due to password protected ZIP files, then you can choose "Settings" from the Boot-time scan section, and uncheck the "Unpack archive files" option. Then it will just ignore all your packed (ZIP, 7Z, etc.) files during a boot-time scan (you can still scan the unencrypted archive files during a regular scan, just put the pastword protected ones in your exclusion list so you don't get error messages for those archives).