Computer hangs on certain game..

Hello,

Intel Pentium D 2.8 Ghz (Single Core),768 MB RAM,Nvidia GeForceFX 5500,Windows XP SP3

I know it's a low-end pc but can someone help me what's the problem? I can play Stronghold 2,Transformers,Over The Hedge,Empire Earth 2,RCT3 without hang but laggish..and I can't seem to play CrossFire,Age of Empire III,C&C3 Tiberium Wars with low specs and even Counter-strike which I can play above 30 FPS ..about 30 minutes playing then the computer hangs...can I repair the pc without replacing it with new hardware?like brush the dust out or something?