Are you up to date with all of your Windows updates?
What is the browser that you use?
Is your version of XP using SP3?
Did you previously have any security software installed ?
Have you done a system restore?
Did your computer recently have any virus or malware that you are aware of?
Have you considered to uninstall via Add/Remove programs and reboot. Go to Useful Links at the top of this page and run the MCPR removal tool to remove any traces of mcAfee that failed to uninstall normally and reboot again. Then login to your online McAfee account and download and install your software.
Not using any beta products by chance? Try MCPR as Tom suggested