... as of June 30th 2009? Where do you sleep? SP3 is out for months!
But then, did you REALLY uninstall ALL other so called security software? Did you really check, like with AutoRuns?
My experiences with MSE are absolutely, totally different. It NEVER used more than 50% CPU on various systems I have it on. Okay, the first scan was like other well known scans but not as bad as e.g. Spybot S&D will slow down my machines while it scans.