As mentioned above sounds like a memory or possibly power problem.
Install
Speedfan amd check stability of Vcore, +3.3, +5 and +12 voltages in the readings tab. If any are dropping significantly below nominal values you may have too weak or failing PSU.
What PSU do you have? Post the +3.3, +5 and +12 Amp ratings (see label on side of PSU). Have you added extra h/w or USB devices recently that would increase power consumption required?
Alternatively try disconnecting/removing all components except graphics card, 1 hard disk, 1 stick of memory and CPU of course.
Try booting from there. If that works add components one at a time (in order of importance to you) until you reach a config that won't boot.
Have you looked in System Information and Device Manager for problem devices?
To check whether it's your memory that's the problem, download
Memtest86 . Install it onto a floppy and then boot from the floppy. The tests will run automatically on boot. Don't be daunted by the GUI, it's very easy to use.
HTH