Given the details that would be my first guess, a bad update. It happens sometimes.
Just what is the matter with MS Security Essentials as far as processor usage is concerned? I used to have it on all three of my machines but it suddenly brought my main machine to its knees hogging the processor after boot-up and it definitely wasn't scanning. Couldn't find a definitive answer, so switched to AVG. In the last couple of days, my secondary older machine has also suddenly ground to a halt after boot-up and the culprit is Security essentials. The only machine it seems to run on OK is my laptop. Dual-core and there is very little processor usage. This does seem to be a common problem related to scanning but I have not got a scheduled scan.