Actually, you don't usually need to replace the fan(s). The ball bearings in the fan(s) probably just need to be oiled. I need to oil all the fans in my workstation about once a year or else they start to make the most annoying noise. Most computer fans have a sticker on them. Just peel back that sticker to reveal the hole where the oil goes. Depending on the fan manufacturer, you might need a tiny screwdriver (like they use for eyeglasses) to open it up to put the oil in. Other times, there's like a rubber stoper that you need to remove. It's not that hard really. You can probably buy a little can of oil especially for CPU fans at CompUSA or Best Buy for less than $10.
Now power supply fans are a little trickier (not to mention dangerous). I wouldn't advise messing with the power supply at all unless the fan for the power supply is very easily accessible. It may not be.
FYI, desktop computers can have upwards of four or more fans. Depending upon the type of CPU and how old it is, there could be a CPU fan. The graphics card will probably have its own fan. The power supply may have its own fan. The case itself will have at least one fan, typically.
