Depends how much you wanna spend m8, as above quieter fans are an option but good ones aint cheap prob most of the noise is down to the cooler fan and HD's if it bothers you that much u can get sound proof material u put on the inside of the case, and if u have loads of money could look @ water cooling zero noise from cpu fan keeps pc a lot cooler... But to do this ur looking at the thick end of 150 quid when u have ur water blocks pipes and reservoir etc, but the bonus is u can cool your GPU too which in turn (if a decent gfx card) WILL improve performance, heat is the enemy to computers... take a look on www.overclockers.co.uk they do a wide range of cooling products and soundproofing
Daz