Although we can uninstall the existing anti-virus using Windows "Add/Remove Programs" (or Programs and Features) applet but its not a perfect solution as sometimes it doesn't remove the program completely which can interfere with the newly installed anti-virus program.
Avast is a little more tricky since it leaves stuff in "C:\WINDOWS\system32\config"
Don't forget about all those registry entries that all of them will create that will be locked in various ControlSet areas of the registry that tools like Revo Uninstaller, Total Uninstall and ZSoft Uninstaller just can't deal with, they have to have the permissions manually changed to Administrator then manually deleted.
Andavari, does the avast uninstaller do the job completely?
It's rather good at getting rid of everything except for the files in "C:\WINDOWS\system32\config" and the registry data in ControlSet areas (that's plural) of the registry. Other than that they have a stellar uninstaller.
Actually every antivirus, and firewall I've ever used places registry entries in the registry ControlSet entries, and not one of them removes them at uninstall time. That registry data doesn't hurt anything, however it's another reason why it's good to track the installation of them or anything for that matter so you know where stuff is, and can at least make sure the uninstall is complete which sometimes details a little manual cleaning.
It might be a ggod time to remind folk that if you are having problems uninstalling a product such as an anti-virus or firewall, boot into safe mode and run the uninstaller from there. Quite often this may mean you won't have to resort to using a specialist removal tool for it.
Also reboot twice after uninstall as this makes sure any of its drivers are uninstalled too.
Microsoft One Care Live install clean up tool (due to be discontinued on 30th June 2009.)
After consulting with the other mods, it's been decided to pin this thread, for obvious reasons.
For now I'll pin it "as is", and give our young poster the credit he deserves for coming up with this, but I think because of it's valuable content, we'll have to clean this thread up shortly, and take out the unneccessary posts, and present it in a more concise way, and even maybe in a more appropriate part of the forum.
That means some of your posts, including mine, will be "cleaned". (Couldn't resist).
If anyone has any objections to that, please let us know.