|Make and model?|
How old is it?
Does it run normally after, of are there other issues?
When pressing F12, does it start immediately, or are you shown an options menu (like F8 or similar)? What options?
Has the laptop been cleaned from dust (internally with a compressed 'air' can)?
Have you kept up with Windows maintenance (disk clean up, browser clean out, etc.)?
Have you tried a System Restore to a date before you began having this problem?
Have you tried a Start Up Repair?
edit: Phil's reply popped up while I was typing. You should only ask the question in one place. If that turns out to be the wrong place, ask the moderator to move the post, do not post the same question again please. One of these posts may be deleted.
You have to be a little bit crazy to keep you from going insane.