beforehand I used to use the likes of McAfee & Norton
did you use a removal tool to remove all leftovers after McAfee and Norton?
you can find it here http://uninstallers.blogspot.com/
If this does not help i would do a new clean install,
using avast removal tool and first uninstalling avast 5 reboot then avast 4.8 reboot and install avast 5 again
I got rid of those 2 a long time ago.
With the Avast Removal Tool when i used it there were tons of options to choose from and uninstall I didn't know what to choose.
There was on the list:
Avast 4 Home/Professional
Avast 4 Server Edition
BART CD Manager
Avast! for Kerio!
Avast! PDA Edition
Avast! 32
Avast! Managed Client
Avast! Distributed Network Manager
Avast! Windows Home Server Edition
Avast! 5 free/pro/internet security
Out of those I first uninstalled Avast! 5 free/pro/internet security, reboot, then uninstalled Avast 4 Home/Professional.
Rebooted my computer and went on to re-install Avast 5 and it still is deleting my MSN whenever I try to turn it on.