I tried out avast! 4.6 antivirus and was very impressed deciding when my current CA e-trust expires to switch. 
However I cannot reinstall my paid for CA antivirus as it keeps detecting
avast and refusing to install despite avast being uninstalled.
I have tried everthing I know including editing the registry, deleting left over program files, system restore and still CA detects avast. >:(
Any suggestions?
Use the Avast uninstall programme from here. Run it then reboot and you should be OK http://www.avast.com/eng/avast_uninstall_util.html
Download Regseeker from this site and check all the registry boxes. Then select “Find in Registry” on the right and search for Avast and every executable file you can remember that Avast used. You may find some in your firewall program. You’ll find that the uninstall programs always leave alot of remnants in the registry which can cause problems. I just uninstalled Avast and did a reinstall, but before I did I removed about 20 items in the registry related to the old Avast installation.
Just be careful not to delete anything of importance that’s not related and to do a backup of your registry before deletion.
Well, there shouldn’t be any important keys left after the uninstaller.
I also suggest to try the removal tool, linked in the second post.