You'd think that McAfee developers would be smart enough to hard-code a list of essential system files into their AV such that it will at least prompt you to get an install disk so it can replace them instead of just deleting them if it suspects they're infected.

For bonus points, make the software realize that even if the file on the CD looks infected, it isn't actually infected, so make that file's SHA-256 sum a 'known good' profile. (If the file on the actual install CD really is infected, that falls into the category of "Problems The AV Can't Solve". At that point, the OS itself is controlled by Malign Forces Working To Destroy You and the game is over.)

For extra special bonus points, code compressed copies of those essential files into the AV software itself, so they can be replaced on the fly without prompting anyone. They can be updated along with the malware profile data, if they ever need to be.