Powerfull PC but…….

hello

P4 2.8 with two 256 DDR 400 Ram On board video ATI Radeon 9100
On XP fully patched

I have the above syetem which flies burning CDs, playing video’s using office and so on. But I seem to crash more than I would expect when playing games. Some games don’t work atall reporting errors regaurding the games EXE file.
Not sure where the fault lies here? is it XP? or a hardware issue? bad memory or video card.
Can anyone sugest any apps that will “test” the PC and give me the results in a way I will understand. Or point me in the right direction here. Very frustrating!!