Things that can slow VirusScan down are as follows...
Is the hard drive compressed? Try uncompressing it. That will improve performance all around.
Do you have a great deal of zipped files? McAfee has to unpack all of those first before scanning.
(There is a thought that perhaps those should be skipped in a future version but I doubt it will happen any time soon).
Check the hard drive for errors.
Go to Start/Run cmd.exe (XP), in Vista/Win 7 just enter cmd in the Search box and right-click cmd appearing above and 'Run as Administrator'
In the Command Prompt windows type in chkdsk c: /r with the spaces and alter c: to whatever letter that drive has if applicable.
You may get an error that the volume is in use try at next reboot? Y/N? Type Y and click Enter and reboot.
Let it do its thing - takes quite a long time depending on the size of the drive.
It's good to do that very occasionally anyway as it irons out a lot of kinks especially if the hard drive is getting on in years.
See if that improves matters.
If not I would open a case with Technical Support Chat linked under Useful Links at the top of this page.
A question....do you have any other security software active?