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