This error could mean that C++ which is required by the avast installation is missing:
Side-By-Side Configuration is Incorrect Error - You need to install MS Visual C++ 2008 Redistributable.
You need 2008 SP1 - this is the one:
Microsoft Visual C++ 2008 SP1 Redistributable Package ATL Security Update
Note this link there are 64bit versions there (IA64, x64) ensure you get the correct one for your OS (x86 32bit OS, x64 64bit OS)