Games crash

When ever I play my two favorite games, Unreal Torn, or Quake 3 Arena, my pc crashes… My pc has the following hardware,

Amd Athlon 900
Creative Labs GTS2
256mb pc 133 ram, 1 20 gig hard drive at 7200 rpm and a 13gig hard drive at 7200 rpm

The games crash after about 20-30min or play, I have Direct X 8 and Open GL 1.1
on the Windows ME operating system.
Any Ideas?