I have been having the same problem.

I believe mine was caused by an external hard drive but I can't be 100% sure. I have however managed to find a roundabout way to get the computer to log on again! (See below for instructions)

http://www.elpassoblog.com/lsassexe-...hutdown-error/

Also, you mentioned running a memtest in the above post. I'm having a few other PC problems that could possible be memory related, could you point me in the right direction with regards to a memtest?

Cheers,
Dan