I have two systems connected via a LAN. One is XP Pro 32-bit. The other is Windows 7 Pro 64-bit.
After upgrading from Avast Internet Security version 5.0.677 to 5.1.864, I discovered that several files and folders were 'missing' on the Windows 7 system when viewing a network share from the XP system. Checking the Windows 7 system itself showed that these files/folders were actually still there, they just could not be seen from the other system. The same problem occurred with other network shares. I checked security access rights and network share permissions (and in fact recreated them) to make sure that there was nothing amiss with these.
Reverting to 5.0.677 on the Windows 7 system cured the problem, but now I am stuck at that level until this problem in Avast 5.1 is fixed.
I contacted Avast, who suggested trying a beta version. I did, but it had exactly the same problem.
To factor out any interaction with other software or hardware, I temporarily reinstalled Windows 7 with no extra software (other than the Intel WiFi adapter driver) or peripheral hardware. The problem reappeared as soon as I installed the beta version of Avast. So I think it looks like a genuine problem with Avast 5.1.