I have the same problem and I'm using only Avast! and nothing else. During uptade I got this error:
ERROR PerformPkgDiff: DSA_FileVerify(C:\Program Files\AVAST Software\Avast\Setup\vps_32-433.vpx) failed, error 0x2000000B
Error:Invalid file signature. Setup will terminate.
Transferred: files 42, bytes 2866891, time 9141 ms
Retries: total 0, files 0, servers 2
Sending stats '
http://46.4.29.3/cgi-bin/iavs4stats.cgi': 00000000 204
NeedReboot=false
Return code: 0x2000000B [Invalid file signature. Setup will terminate.]
Stopped: 16.03.2011, 10:02:45