Answer
This question was answered on April 5, 2006.
Much of the information contained herein may have changed since posting.
There are many possible answers. The answer that is mostly likely the cause of the problem is either a virus or a software conflict. If you can, start your computer up in safe mode, this allows the computer to run on the bare minimum programs it needs to run. If you can get the computer to start in safe mode, try running your anti virus software software to rule out a virus causing the problem. If you run anti virus and still have a problem booting your computer up, you may want to consider booting into safe mode again and remove any programs you added right before you encountered this problem. Best of luck to you.