- Norton, Avg, AOL - how many do you want? If you have ever 'uninstalled' one of these, have a trawl through your registry and see.
I've uninstalled all of them and have never had a problem. They are part of the often-preinstalled junk that I routinely throw away when setting up a new PC.
I've also installed Microsoft Security Essentials after removing the above products, with no problems. You have to use the official uninstall programs.
Looking through the registry doesn't tell me much, since the ways in which the registry is used are proprietary to each software product. If I didn't write the code, it can be hard to figure out what the product changed. Therefore it is best left alone. Conflicts are theoretically possible, but I don't recall having any, although sometimes one version of a product will conflict with another version of that product (but if the product can't even stay clean within itself, why would you trust a third-party registry "cleaner" to do it?).