When you say it doesn't boot, do you mean you don't get anything up on your screen?
try resetting the CMOS by either moving the jumper from pins 2/3 to pins 1/2 or by removing the battery for about 30 mins. Then try to boot up.
If you're getting the initially words and stuff on your screen, but it's just Windows that isn't booting, then:
a) if you have to backup files, put your drive in another computer and copy your files
b) reformat your computer (do a fresh install of windows)