I assume that by McAfee you are talking about VirusScan.
Please install the latest VirusScan patch, reboot the computer and try to run the scan again.
PS: What version of VirusScan did you install? Are you running XP 64 bit?
Could you please provide the following additional information likke;
What is the operating system you use ?
What is the version of virus scan & DAT (Open security center & click about)
How old is the computer ?
Do you receive any anonymous pop ups while browsing the internet ?