It sounds to me like the software has been corrupted somehow. I assume this is Windows 7 SP1 and fully updated via Windows Update?
Try uninstalling McAfee from Control Panel > Programs > Uninstall a Program, then running the MCPR cleanup tool listed under Useful Links at the top of this page, reboot and reinstall from your online account.
Ex-Brit, tried that and still same problem when trying a full scan, ok on quick scan. I still think there is a McAfee/Windows 7 problem as I found it very strange to get a message form McAfee last Friday tellin me to urgently download an update from Windows?
I find that strange too unless you ran the Vulnerability Scanner that's found on the latest version of the products. Did you install the update it suggested and are you totally up to date inclkuding parts you may not use such as IE?
I suggest Technical Support, it's free and avail;able by phone or online chat and linked under Useful Links at the top of this page.