McAfee was and still is, IMO, crap. I recently installed it on a clients machine because I had a free license and wasn't going to use it. After rebooting all kinds of application errors were occuring. Even after I managed to download the latest patches. Uninstalled, system was fine--not a glitch. Installed NSW2K2 and not a single problem again. As mentioned above, McAfee is a pile.
Incidentally, That was my experience a few years ago and why I had given up on McAfee for so long. I remember when they were the only free program and were well designed (back when it was a dos commandline prog). Since then, it sucks. I managed to get a free license a bit ago and had held back on installing it. I kept it just in case and then had the above experience.
For Enterprise A/V I hear good things about Trend micro. Followed by Symantec. (and everything else). and then McAfee/NAI.
YMMV.