Avast! refused to let me install the following from Microsoft.
Security Update for Microsoft .NET Framework 3.5.1 on Windows 7 and Windows Server 2008 R2 SP1 for x64-based Systems (KB2943357).
I had to install this while virus protection was disabled. At least once it was installed, Avast! did not complain.
Program version: 2014.9.0.2021
Virus definitions version: 140812-3
OS - Windows 7 Home premium 64 bit