When you say "the lights come on", are you refering to the Num Lock, Caps Lock and Scroll Lock lights?
If that's the case, can you tel us whitch are on/off/flashing?
Several laptops use those to report the progress of the POST phase. If it don't finish normaly, the code can inform you about at what step a problem occured and interupted the boot process.
For example, ther is a code to report a RAM problem, another to report a non-functioning or faulty drive, then another if there is a problem with the video,...
If the POST finishes normaly, you probably have some missing or corrupted system file. In this case, you can boot from an install CD/DVD and perform a repair of your installation. It should preserve all your data and installed programms. It WILL revert any and all OS update.
Electro
July 2012
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix notebooks/laptop pcs.