Have you opened up the system unit to see if the inside is very dusty? If it is, then the fan may be making more noise than it might because it is clogged with dust. Carefully vacuum out the dust and see if this makes any difference.
If the noise is being caused by either hard disk or cd rom activity then there is probably little you can do. If either is failing you could choose to replace them, but I wouldn't recommend trying to insulate them because cooling is important. Air must be able to circulate around the components.
Consider noise when buying your next PC. Integration of music & video may mean you'll want your next PC in your living room. Microsoft are developing Windows Media OS for just such a purpose. PC makers are soon going to find consumers want quite machines.
Keith H, May 2004