Recently replaced power supply, computer is working but seems slow - what is likely to be the problem?

The computer just refused to turn on one day, I presumed it was the power supply so took it out and then replaced it with a new one. The old one was 300W and the new one is 400W, but I don't believe this should cause any problems. The graphics are jerky, even in something simple like winamp visualiser. If anyone can tell me what is wrong,or how I can find out I'll be very grateful.