Computer freezes up

hello. my computer has this problem, where after playing a game of any kind, it just freezes up. i have tried installing new drivers for my video card, turning off 3d acceleration, turning AGP from 4x to 1x, reformatting, and none has worked. my computer only freezes when i play a game. please help.

my system is:
Windows 98se
AMD Duron 800 mhz
128mb PC-100 SDRAM
NVidia 32mb RIVA TNT2 m64/m64 pro
um…some sound card..whatever came with my computer

also, i have to restart my computer if i want the sound to work. if i start it up, the sound doesn’t work. i have to restart it so that it works. what could be wrong?