I found that nothing fixed this problem---not complete uninstalls, reinstalls, installing "special" files, renaming files, etc., etc.
The only true fix I found was to do a complete uninstall of McAfee Security Center 2009 and install Symantec's Norton AntiVirus 2010. Voile, no error messages, consistent protection, and no spending a lot of time tinkering with things to get them to work as they're supposed to work (and still not having them work right). As far as I'm concerned, McAfee has gone down the tubes as far as product and official tech support (although the volunteers here truly try their best to help).
I've posted this on a couple of threads because so many of us are experiencing the exact same problem with no "official" response or fix from McAfee. I think that's unacceptable, I paid to have computer protection and it isn't being provided as advertised by McAfee. I won't risk the security of my PC or laptop, it's not worth the price of the unused subscription.
I highly suggest this "fix," it saves a world of headaches!