need help.pc problem

[bobrik75]

hello.i need your help.
please tel me what is it?
i buy new PC dual core pentium 3,4 mit GE forse 7900 gt.
i was playing some game,and for a few minutes go away from pc.when i come back on my momitor was black picture,whith som sort of lines.i try t reset pc but nathing,then i try to switch off pc but cant do it.
i take off it from power.than open the pc and take my video card out from pc,then put it back.startet pc again.and its work!!!
i was thinkihg that is my card was bad instaliren,but today when i start the game its hapend again.onlY this tame its startat working again after i restartet pc.after i was playing about 3-4 hours and ales where gutt,but i afraid this happend again,please tell mi what is it,where is the problem.
Thanks