McAfee relies heavily on a working and up to date Internet Explorer. Go to IE Tools/Internet Options/Advanced and click "Reset", OK any prompts and click Apply and OK. Close and reopen IE, go to Tools/Manage Add-ons and re-enable all the add-ons.
(This is assuming IE7 or 8 which you should have). Whether or not you use IE this still applies.
Try running MCPR in my signature again, reboot.
Run the McAfee Preinstall Tool from the following link:
http://download.mcafee.com/products/licensed/cust_support_patches/McPreInstall.exe and attempt to reinstall.
Another factor that can prevent installation is competing software....is/was there any?