If you are talking about the fan built into the power supply DO NOT remove it or open the power supply. Even though it is disconnected from the wall it can still hold a charge powerful enough to hurt you. Buy a new power supply do not attempt to replace the fan in the power supply. As far as the computer fans you, can purchase replacement fans that are really quite look for a fan with a low dBA (noise level) and a high cfm (air flow). http://www.newegg.com/ProductSort/SubCategory.asp?SubCategory=62
AJZ