This could be one of several things. First, attempt to get into safe mode. When you start your computer, shortly after the POST (the black screen with white letters) press f8 until you see a message with several options. one will be safe mode. use the arrows to get to safe mode and hit enter.
If you get into windows, then check your startup folder. Also read up on how to do a clean boot through the MSConfig command. Then try getting into windows normal.