Safe Mode. You could also try going via "last known good configuration" in same menu. How to start in Safe Mode:
http://www.pchell.com/support/safemode.shtml

My Temptation would be to go SFC route first. Please let us know results.

How old machine & HD? What AV & Anti-malware have you (exact versions & last updates, please)& anything else weird happening. Happy