My Windows XP installation was acting weird, executables were not working properly, so I reinstalled XP. Of course I immediately reinstalled Avast, because safety is good to have. However, whilst trying to reinstall my other software, I would have problems installing new software. The software would give random errors throughout the installation process, usually “install error 2503,” but other problems would happen. When trying to run a program, it would give several errors of being unable to start. Clicking the program several times would make it finally execute.
After much frustration and hair-pulling, I narrowed it down to Avast’s file system shield. Within the settings, I tried to individually disable “Scan X while executing,” ending with unchecking all three boxes, still yielded same errors. Fully disabling the file system shield allows things to execute and install properly. Latest version, 2014.9.0.2013 installed. :o
Sounds like you may want to do a full clean uninstall & re-install of Avast.
How-To: http://forum.avast.com/index.php?topic=143284.0