Games will freeze while playing

I have just bought a new Athalon 750 with a Riva TNT2 M64 video card with 32 meg memory on card. My problem is that on boot up right before the icons load it will freeze the screen and I will have to reset the pc. Also when I am playing a high graphic game it will sometimes freeze up while i’m playing and the only thing I can do is reset the pc.
Any suggestions what could cause this?
I am running win98 se on a thunderbird 750 amd with 128 meg memory