Can you hear or feel the hard drive spinning? Can you boot to the BIOS?

 

Start the computer boot into Setup (Bios) here on the Main window it will show all of the drives. Is your HDD listed? If not, it has failed. As Chief asked, when the computer powers on, put your ear next to the HDD, can you hear or feel it spinning up?
You can make an Ubuntu Live CD or USB and boot the computer with that. Then you can Try Ubuntu then browse the C: drive and copy your personal files from there to a USB HDD.
Do you have a Windows CD? If the HDD is spinning, After safely backing up. boot off of the XP CD and Reinstall Windows.