SP2...Vista or XP? How much memory do you have installed (RAM)?
There could be three factors at play here - insufficient memory, infection or leftovers from another installation.
First, enable Windows Firewall to provide you some protection.
Check for spyware/,alware using the free version of these two tools, update them before running and let them remove anything they find. http://www.superantispyware.com/superantispywarefreevspro.html http://www.malwarebytes.org/mbam.php
If that fails then try running the MCPR removal tool linked in my signature & reboot and retry.
If it indicates that it was unsuccessful run it again and reboot then retry.
Another thing that sometimes helps, assuming you have either IE7 or 8 installed (you should have 8 even if you don't use it) - go to IE Tools/Internet Options/Advanced and click the Reset button. OK any prompts and click Apply and OK.
Close and reopen IE and go to Tools/Manage Add-Ons and re-enable the add-ons.
McAfee relies on an up to date IE working at its default settings.