I read this old thread with interest, because I have been having the same problem as Alex. Although he received helpful advice to enable him to look at the results of scans, I don't think that his basic problem was solved; i.e. that there was a probably a fault that was causing all his local drives to be scanned, even when he had carefully selected only the drive(s) that he wanted to be custom-scanned.
I have reported the same problem on the BT Care Forum, since my McAfee system came as part of BT's broadband Option 3 package. No-one has yet come up with a solution, although I have seen advice elsewhere that the only solution is to re-install the entire McAfee system.
In the meantime, the only simple option that I know is as follows: Open Windows Explorer (or "My computer") and right-click whichever drive or folder that you want to scan. Then select "scan" from the drop-down menu. This works fine for me, except that there is no obvious way of knowing for certain whether the scan has included all categories of threat. I will, however, see if I can do so by lookig up the log in the advanced menu, as Alex was advised to do.