1. Only run ONE interactive virus scanner.
2. Only run one software firewall.
3. Disable all startup programs that you do not immediately need. Start things when you need them, then close them when you have finished.
4. Defragment your HDD in safe mode (that defragments the virus signature files ).
5. Disable the scan on file closure option.

Check your running processes, particularly automatic updates.

McAfee and Norton are rather resource hogs, however you may have conflicts. The best policy is to run as little as possible at any one time.

Check for program updates. AV and operating system.



EDIT: And if you disable your AV, you might as well not have it?