TNT2 Ultra crashes during DirectX game play

I am experiencing some nasty machine crashes during DirectX game play.
Playing 'Rise of Nations demo' , all may go well for an hour or more, then
suddenly the black screen of death and the machine restarts. Playing 'Empire
Earth', I get all the way through to the tutorial START button, but when I
press it, nothing happens. Then when I press another key, the machine
restarts.

Dxdiag shows no issues. I am using Dx9.0a at this stage (installed from the
SDK redist). The PC is a Packard Bell PIII-650 with 128 MB RAM, nVidia TNT2
Ultra 32MB graphics card running latest Detonator drivers, Yamaha Xwave
sound card, PCI LAN card, 3.5 GB free disk space, VM set at 324 MB min/max.
Not sure if any other hardware info is relevant.

Any help or pointers would be appreciated.

Tony