If they were truly found by the network shield then they aren't files on your system, so it isn't any surprise that you didn't find anything. The network shield blocks access to malicious sites or attempts by malware to access your system using normal exploit ports.
Check the Network Shield, Customize, Last Attacks tab, see image example, this lists these attacks/attempts.
If they were detected in the normal scans then this would be what to look for:
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe