Fan replacement is easy! Be brave and open the case; there's very little chance you will damage anything unless you have the dexterity of a gorilla.
1. Go find a medium phillips head screwdriver. Newer consumer cases usually have tool-free cases, but you'll need it anyway to remove the fan in a few minutes.
2. Remove the power cable first, along with every other cable (go ahead and draw a picture of the back of the computer and label every cable with tape if you are worried about hooking them all back up correctly).
3. Open the case. There are only a handful of machines that defy patient logic.
4. Find the fan. It usually has 2 to 4 screws holding its enclosure in, and one or two cables. Take a photo of exactly where each cable plugs into the power supply and/or motherboard if you believe you may forget. If the fan is PART OF the powersupply (where the power cord plugs in) then you'll need to replace the whole power supply.
5. Unplug the fan cables and unscrew the fan enclosure's 2 to 4 screws. Keep the screws safe because new fans don't always come with the right screws.
6. Bring the old fan to CompUSA or whereever and tell the punk in the red shirt you need to replace it. Neither get the cheapest NOR pay more than $25 for a replacement fan unless you know what you are doing. Make sure the new cables look like the old cables.
7. Put the new fan in whereever you took the old fan out. Plug in the new cables.
8. Close the case. Reconnect all monitor, speaker, keyboard, mouse, printer, network, USB cables. Reconnect power cable.
9. Restart and enjoy the tranquil quiet!