Why i can't maxed out gta iv and crysis warhead?

Just run the games in 1024x768 and see how well it runs. If it still runs slow then there is a problem. Try updating you GPU to see if it gets a boosted performance if it still is slow @ 1024x768 then virus sweep just in case there is a virus. If the games still run slow then there is a definite problem with your PC.