It would be helpful to know some info concerning your computer such as operating system, ram, type of internet connection, etc.
You could try running McAfee Virtual technician found under Useful links at the top of this page in the Technical Support area. MVT will scan your computer and attempt to fix any McAfee related issues it may find.
You could also try to uninstall and reinstall the software as something may have become corrupted. If you do this be sure to run the MCPR removal tool before you reinstall.
If no joy then contact Technical Support as they have more tools available to troubleshoot problems.
Also big cause is old Avs still installed or not cleaned out correctly. Did you have another AV on the PC before Mcafee?