Situation 1: You're infected. Probably not the case here, but have you also tried SAS (free), an online scan with A-Squared and an online scan with another AV like Kaspersky ? You may have to disable McAfee's real-time protection for the latter, I'm not sure.
While it probably would not help, you might want to try this in case you got a corrupted DAT download: http://forums.mcafeehelp.com/showthread.php?t=229135
You have tried uninstalling and reinstalling. Did you use the MCPR tool and the other instructions that can be found on this forum ?
Is it possible that there is something on your harddisk (like software, images) that could cause this ?
A possibility is harddrive failure. I think you can fix/check this with CHKDSK.
The question is, why did it work for a few days and then stopped ? You have a 3 user download, is your computer perhaps in any network, and could this somehow (malware ? McAfee settings ? ) cause your problem (maybe disconnecting it for the scan would work) ?
Often a plain reformat followed by reinstalling everything else is the best option.
You might not want to use McAfee after that ...