games freezes after playing for a while

[J_genius]

for all the games i have (both backed up ones and origenal ones), afetr i play it for a while, some games's sound stops and some other just freezes. For the ones the the sound stops, after a while it too freezes. When it freezes, i can do nothing about it. nothing works except pushing the reset bottom on my comp case.
I have xp 2.6ghz, 1/2 Gb ram, radeon 9600