Could be a lot of different things, among them, the power supply, faulty Ram, recently installed hardware and or software.
To check the Ram, download and run "Memtest". You will need to unzip it and "extract" to a floppy. After that reboot with the floppy in place.
You should also be checking for properly seated Ram, and or video card. Pay particular attention to the cleanliness of the interior of your machine.
Dust Bunnies play havock! Inspect closely your CPU cooling fan and heatsink as well as the power supply.
Memtest!