you need to put in the system restore disk or the windows operating system disk that came with the laptop when you bought it.
If you have a warranty, call HP and tell them what has happened. If not, use your Windows OS disc to repair the operating system. You will most likely lose all of your files though.
you would have to call HP/Compaq and tell them to send you the disc for your operating system Vista/7. If it is still under warranty they will probably end up charging you about ten bucks as this issue is not covered by the warranty. ( Ive had this problem before)