my pc keeps freezing?

hi,my pc keeps freezing when playing games useing my games card so i bought a new one (a radeon £155 bran new) and the problem keeps happening.i installed more ram and i have a 1.7 gb proc...It still freezes and then the other day i was only playing music using windowmedia player and it freezes again without running any games in background,there has been time where it dont crash for hours and sometimes crashs after 5 mins.it seems to happen more and more often...has anyone have in answers...plz plz plz...???