computer lock up while playing games

i have a gig of ram 128 megabite vidio card extasy 9200 americas army,
countersrike& day of defeat keep locking my computer up after about 5 minutes
of play i did every thing my help center said. i turned off all security
start up programs every thing . i'm running windows xp my computer is a year
old every thing else on computer runs great and this happens playing off line
if any one could help me i would apreciate it?