Is this a new system ?
If so is it not possible that it can configured with an anti-virus or other security software ?
Perhaps I’m reading something into this “Avast was the first one I tried to install” did you try to install anything else ?
Are there any other errors, install failed is somewhat brief, check the windows event viewer (I don’t know if this is what it is called in Vista) and see if there are any errors relating to this.
I’m also wondering if this is a Vista UAC thing, are you installing with administrator privileges ?