The gzip files may or may not be corrupted. Avast just may not be able to unpack them.
Schedule a boottime scan, reboot. See if avast finds any thing.
Also download. undate and run avg antispyware (formaly ewido) preferably in safe mode.
http://free.grisoft.com/doc/20/lng/us/tpl/v5
or superantispyware http://www.superantispyware.com/
edit to add you may also want to disable system restore
Post back your results.