You may want to boot into safemode and edit the startup through msconfig. What you would do is goto start > run > type "msconfig" and it will bring up a window. Click on the startup tab then you may want to start unchecking items and restart your computer to see if that makes a difference. Also, have you recently installed any software prior to this problem? If so, you may want to uninstall it through safemode. Just a thought.