Computer keeps freezing



my computer keeps freezin up on me and by freezing meanin a still screen, not turnin off just stops…i thought it was the ram so i replaced it though i had 2 sticks of 1g now im down to 1g…i recently reformated and added a new HHD…was my friends HDD so i know it works…mostly when i play a game i can suff the web for hours then i play a game and it freezes also sometimes at statup…idk what else to i tired everythin plz help…im runnin AMD athlon 64X2 duel core processoe 5200+, 2.71GHZ,1gb of ram with a nvidia 7200 video card with a 100g HHD