Here is an overview of what you should try, before asking further questions:
Testing your memory:
Bad memory is often the reason for random crashes. In order to test your memory download http://www.memtest86.com/
and make a bootable cd or disk. Start your system from that medium to run the memory test.
It is probably enough to run it for one pass. Look at the lower table that looks like this after one pass is finished:
Test Pass Errors
---- ---- ------
Std 1 0
If there are errors they are showed in red as in this example:
Testing your graphics card:
Open your PC and make sure the graphics card cooler is dust free and look whether the fan runs when the PC is running.
to stress test your graphics card. Start it, select "Stability Test" and "Xtreme Burning Mode" and press "Go!".
While running you should monitor the temperature at the bottom of the screen. Run it for 10 minutes or until the temperature seems to be stable for some minutes. If the temperature exceeds 80°C, abort with Escape.rature seems to be stable for some minutes. If the temperature exceeds 80°C, abort with Escape.