Several possibilities a big 1 would be you may have a virus. That said try the following first before we look at the virus possibility
1. Have you any other AV programs installed or had been install prior to this version of Mcafee.? This includes other versions of Mcafee. If you have or had you may nee dto run the relevant removal tool for the program you had. If you had Mcafee (aother version) yopu should have run MCPR removal tool mentioned here
2. try a manual update somethimes this shocks it into working.
these ideas and other thoughts mentioned here
Moved out of the SiteAdvisor section into Security Center
There are several issues for the Real time scanner to be disabled , some of them are ;
If you performed a system restore
Updates not installed
Conflict with a third party scanner
McAfee services not started
We have a propose document here http://service.mcafee.com/faqdocument.aspx?id=TS101225 that would help you in troubleshooting and fixing the issue without the intervention of a technician , please try it and let us know if you still have the issues persisting ???