i would simply vote McAfee SecurityCenter as the worst Antivirus of the year... stay away from this crappy AV because:-
1. it is resource hungry
2. it will not allow you to exclude a certain files or folders from being scan !!!!
Reason no 2 will be enough to get you kill because certain files and folders are meant to be excluded from being scan....just imagine if you are running virtual machine and the AV keep on scanning your VM client files..wtf!!
A simple google on the net return results indicating this is an old issue which McAfee have not fix till today...
If you want to have fast performance for your Windows Vista or Windows 7 .. stay away from McAfee SecurityCenter .. in my case my alienware m11x come with this lousy AV and now i have to uninstall it... maybe of asking dell for a refund of this AV..