Well, i think it has to do with the compilation of Avast during installation, some spawning errors occurs when in Visual Studio you try to compile with C++ , and you dont have the compiler installed. Maybe, there is something missing on your systems, maybe .Net framework update, a Visual Studio update. Because Avast installs an update for Visual Studio Redistributable, called Vcredist x86, or something like that. Maybe you need to download the package befor installing Avast 5, from microsoft update, or enable that feature in your win xp, so you can have all your windows enviroment up to date.
Well, i think is that, hope this info can help you pal. take care.