I finally got around to replacing the CPU cooling fan in my main computer with a considerably larger cooler.  Bigger cooling fans in the computer can move the same or more air without spinning as fast, so they are quieter.  I ended up replacing a small "stock parts" type CPU cooler that had about a 3 inch muffin type fan with a much larger cooler with two 5 inch fans.   

The reduction in noise was considerable.  I can barely hear the computer running now, even in warm weather.

Recording is a high demand activity on a computer.  It makes the CPU work much harder than checking your email or watching a silly cat video on youtube.  When the cpu works harder, it gets hotter, and one of the things that happens is your cooling fan can kick into high gear.  Which is more noise, if you are recording near the computer.

Most home computers are not really designed to be good for recording.  (Yes Mac users, I mean most Macs, too.)  They aren't designed to be especially quiet or have as good a soundcard as you'd want for anything like pro results, like the sort of computers in a state of the art recording studio. 

That doesn't mean you can't record on stock home computers these days and get some very good results.  It just means there is a certain degree of compromise.  You may need to use noise reduction software to clean up the bit of noise from the fans, and you won't get quite as good results when trying to do a lot of tracks or FX as a professional machine, even if you happen to be running the same exact software.

Upgrading the cooling system in your computer (or having it done at a computer shop, if you are the sort that doesn't do such things yourself) is one option for making the machine quieter and usually getting a little more performance out of it.  By itself, it won't make your basic home mom&pop computer into a pro audio workstation, but it can get you just a bit closer.