Boot-Up Stopping on W-XP Screen.

What would cause the computer to get hung up on the XP Screen during Boot? The Compaq screen comes on, then it goes to the XP screen and freezes. Then I have to unplug the tower, and plug it back. The only way I can get around the freeze-up, is to hit F8 before the xp screen, then click on "Most Recent Configuration That Worked ". This problem is preventing me from being able to Restart my computer when required to do so, like after updates and such.