Looking at this page: http://pcsupport.about.com/od/inside...owitfits_2.htm
This is a PC with the side wall off, that's all you should need to remove from the case - there'll be screws at the back on the opposite side to where everything's plugged in.
The big board that everything is mounted on is the motherboard. On my machine, the silvery square bit just below the huge fan had a smaller fan on it and that had gone slightly skewed and was making a noise. Not all motherboards have this smaller chipset fan, though bear in mind it might be the CPU fan (the huge one in the picture), there may also be a fan at the back of the case and possibly on the front as well, and also a fan in the bottom of the power supply (the big block at the top of the case).
You can (relatively) safely go inside it when it's running, this will help to determine where the noise is coming from, though avoid touching the power supply and if you're going to poke anything make sure you touch the case first - this will remove any static electricity you may have built up and stop things from breaking. Obviously turn the power off before you make any adjustments, you should only need it on to find the source of the noise in any case.
Hope that helps,