Vista - MsMpEng.exe spiking 100% CPU - temps 90+

I installed MS Security Essentials Beta and then Vista x64 SP2... ran defrag and used auslogics registry defragger. No other changes.

My CPU spiked and the temps climbed so rapidly, I had to shut it down and put it over the a/c vent to cool it off. On reboot, they immediately spiked again.

Booted to safe mode and it wouldn't allow me to uninstall MS Essentials. Boot to Windows > temp spikes > cpu loaded, but I managed to get into CP to uninstall MS Essentials.

Temps immediately went down. I reinstalled Comodo AV for protection until I figure out what happened.

Googling MsMPEng.exe tells me it's Windows Defender. I did see that checked while in msconfig in safe mode. I thought MS Essentials was supposed to disable that??

Can anyone tell me what happened? I'm on a laptop and am scared to load MS Essentials again. Especially if it uses the Live Care One/Windows Defender MSMPENG.

temps scared the crap out of me!