put the recovery CD into the CD-rom, reboot computer. There'll be too options, erase everything on your disk and install OS fresh or do a repair reinstall. I'd recommend doing the first option, but before you do, make sure you backed up all your important files because everything will be erased on the computer.
alternatively there's a recovery console on HP computers (can be accessed by hitting a key during start up read the screen prompts) recovery console will let you repair the OS