The 2 main things that would wipe out McAfee directories are malware or registry cleaners. The latter are notorious for doing that.
Try running the Virtual Technician to see if it will fix it, although I doubt it will: http://mvt.mcafee.com/
If not uninstall via Control Panel/Add or Remove POrograms, then run MCPR cleanup listed under Useful Links at the top of this page.
Reboot and then download again from your online account.
Also might not be a bad idea to run Stinger and Malwarebytes Free, listed in the last link in my signature below.
By the way I trust this is XP SP3?