It sound to me like it's corrupted somehow.
Yes it is possible to download both the engine and the DAT antivirus updates directly. See: https://community.mcafee.com/docs/DOC-2155
However I think you'll be best served by uninstalling completely via Control Panel > Add or Remove Programs.
The download the MCPR cleanup tool linked under Useful Links at the top of this page and reboot.
Then run the McAfee Preinstall tool which prepared the machine: http://download.mcafee.com/products/licensed/cust_support_patches/Mcpreinstall.e xe
The go to your online account and install the product again.
A hint, if you are in the habit of using System Restore a lot, always run updates to McAfee and your system immediately afterwards otherwise things can go weird. Also if you are using Registry Cleaner, Optimizers, Tune-Up Utilities etc. be really careful as we have found them responsible for many of the problems people have with malfunctioning software and systems.