Try booting to Safe Mode and deleting the contents of the C:\Program Files\McAfee\VirusScan\Dat folder, reboot and right-click the tasdkbar icon to update immediately. The replacement (good) update is on the servers.
If that fails try System Restore then update immediately.
I'm assuming you mean 21/04/10
Thanks for your reply.
I booted in safemode, searched in the McAfee program files folder, but there was no Virus scan sub-folder, and the only Dat files (in other McAfee sub-folders) were:
Any other ideas?