Avoid enclosing the PC
A lot of desks are designed more for aesthetics than functionality.
Some desks include a computer cabinet that are designed to keep the Computer out of sight. The problem with these cabinets is that they prevent air from circulating. As a result, the computer keeps recycling the same air, heating it a bit more each time it passes through the case. The temperature may eventually rise to the point that it gets incrediby hot inside the computer case and this is when the fans may get very very loud and components can begin to fail.
Allow for Air Flow
The easiest thing you can do to help keep your computer cool is to give it a little breathing room by removing any obstacles to air flow.
Make sure there's nothing sitting right up against the sides of the computer, especially the back.
Most of the hot air flows out of the back of the computer case. There should be at least 10-20cm clearance on either side and the back should be completely open and unobstructed.
Remember Cool Air comes in through the front and the hot air is extracted through the back and sometimes the top as in the above picture.