b> WARNING: Before you start troubleshooting remember that you are dealing with electricity that can KILL. http://www.kitchentablecomputers.com/static.php - rules
Only work inside the computer case when the power has been switched off and disconnected. Never open the power source. Some of the below steps recommend removing physical parts within the computer. While in the computer it is highly recommend that you be aware of ESD and its potential hazards Every computer comes with its own internal cooling system. Dell workstation desktops are no different. Your computer's cooling system consists of a heat sink and cooling fan. The function of the cooling fan is to regulate the temperature of your computer's processor. If the fan stops working your processor will overheat and cause damage to your motherboard. Fortunately, Dell desktops have a built-in alert system that tells you when your fan stops working.
Shut down your desktop computer. Disconnect the power cable from the computer's back panel. Then disconnect any external devices. Rotate the computer so the back panel is facing you. Attach an electrostatic discharge (ESD) strap to your wrist and its wire clip to the outer casing of the computer. Lay the computer tower on its left side. Press on the side panel release buttons situated on the top and bottom panels. Grasp the side panel and rotate it away from the computer until the internal contents of the computer are accessible. Locate the cooling fan duct near the center of the motherboard. Lift the duct off of the cooling fan so it is in the upright position. Disconnect the fan's power cable from the motherboard. Disengage and remove the retaining clips from each side of the cooling fan assembly. Lift the cooling fan out of the motherboard. Set it aside. Check you CPU make sure its securely seated and has thermal grease http://www.hardwaresecrets.com/article/Thermal-Compound-Roundup-February-2012/1490
Use a clean paper towel to wipe the excess thermal grease from the processor's surface. Apply a thin coat of thermal grease to the processor's surface using a cotton swab. This is important; if you skip this step, your processor will fry as soon as you power on your computer. Place the new cooling fan over the processor. Press down on each retaining clip until the cooling fan snaps into place. Connect the fan's cable to the motherboard. Place the air duct back over the cooling fan assembly. Rotate the side access panel downward over the computer's opening. Press down on the side access panel until it snaps into place. Place the computer vertically, and then reconnect all cables and devices to the computer.