Sometimes depending on how far it will boot and what version of windows it is. You can hit the F8 key as it is first starting up and get into safe mode. From there you can get into "My Computer" and right click on the "c" select the tools tab and do a defrag and schedule a disk check - check both boxes in disk check - one says "check for errors" the other one say "attempt to repair"
This might fix it about 60% of the time.
Sometimes it is caused by the device driver errors.