DavidR
11
If you are having problems installing other AVs it could well be a rootkit killing them.
See http://forum.avast.com/index.php?topic=26554.0
http://forum.avast.com/index.php?topic=25941.0
http://research.pandasoftware.com/blogs/research/archive/2006/12/14/Rootkit-cleaner.aspx
http://www.f-secure.com/blacklight/try_blacklight.html
What were the problems in trying to install or run these ?
You need to exercise care if you installed NAV this can be very difficult to remove at the best of times and when you clearly have a problem possibly more so.
When you click the Change/Remove you should get a pop-up with other options, see image.
http://img.photobucket.com/albums/v325/for-dwr/avast-repair.jpg