Isolate which is noisiest component then go about maing it quieter, if its generally noisy then its prob case fans want replacing, after that pinpoint wich component, prob gfx card, could add aftermarket cooler to this, carry on with each noisyest component til it gets to an acceptable level, you can also add dampening material to case, suspend HDDs using elastic, loads of things to try, depends how far you want to go