I'd start with checking if the PC will boot from a bootable disk. First, check in the BIOS setup if it's set to boot from the CD/DVD-drive. Then insert such a disk. A good disk to try is the Windows XP setup disk, because it allows you to do a chkdsk from the recovery console.
But any other bootable disk you can find (or download and burn) is good also. Especially if you choose a Linux disk (Ubuntu is the usual choice), because you can browse the hard disk then to see if all your files still are there.
But I wouldn't be suprised if there is some hardware issue and it won't succesfully boot from a CD or (if it does) report a hard disk error if you try to access the hard disk.
I am running XPpro and Security Essentials protection. After 21 Windows updates and Security Essentials update last week I did a full antivirus scan. But the computer turned itself off and now won't boot not even safe mode. I just see alternate accesses to the hard drive and the CDROM. The screen is blank, can anyone help?