Test your PSU power supply unit or replace it if your power supply units fan is not working your PSU is faulty http://pcsupport.about.com/od/toolsofthetrade/f/powersupplytest.htm
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 Remove the memory modules from their slots. Take the opportunity to clean the slots on the motherboards and the memory module connectors.
Use compressed air to blow dust away and clean contacts with a soft cloth. Do not use a metallic vacuum cleaner if it touches any component it may create a short and cause damage to the motherboard or other components. Do not use solvent that may attract dust and never poke things like cotton buds in to slots.
lightly rub an alcohol on a cloth not to hard let the alcohol do the work on the copper gold or silver tabs and r Allow the pins to dry. They will air dry in a matter of minutes. If reseating the memory did not resolve the issue try swapping the location of the memory. If you have only one stick of memory in the computer try moving it to another slot and then boot the computer. Check the memory module and memory slot contacts. They are either copper tin or gold. The colour will tell you which they are. Mixing tin and gold can result in corrosion that prevents proper contact. Look for any sign of physical damage to the memory module, memory slots or the motherboard. Reseat the memory modules. You should hear an audible click when they are in place. Do not use too much force to reseat the memory module in to the slot this can cause damage to the module, slot or motherboard. The hardware that you are trying to access is damaged or failing.
The device drivers for the hardware are faulty or incompatible.
Could be a problem with the Hard Drive or the Hard drives PCB http://www.onepcbsolution.com/
There is a connection problem such as a bad cable for internal hard drives
Test all power and data leads that attach to your hard drive IDE,SATA
the leads from your MOTHERBOARD TO THE HARD DRIVE make sure they have a
secure dust free connections and are not faulty or just replace them they could be faulty if its a flat 40pin ide this will be the first to fail Make sure all leads that are attached to your drives dvd\cd and floppy (If Applicable) have secure connections and are not faulty. or just replace them they could be faulty a computer needs power and data to travel through every working device and continue its cycle and have an end so any faulty leads will end up with a computer error
Hope this helps.