MSE
Nope.
1. It's free. Why would it be asking for registration and halting all other antivirus programs?
2. It's not default. I've owned 5 Windows 7 computers so far and have helped with other peoples' computers running Windows 7 as well and none of them came with MSE.
He's quite obviously getting a fake anti-virus called Windows 7 Anti Virus or whatever, and it's stopped use of his real AV so he can't remove it.
I'm going to say do what uno suggested, but do it in safe mode.