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 One bad lead can cause a computer to continue on a cycle of restarting or to shutdown or fail to detect/ boot up a computer hard drive 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 Test all leads that attach to your hard drive power and data cables IDE SATA the leads from your
MOTHERBOARD TO YOUR HARD DRIVE make sure they have a secure dust free connections and are not faulty or just replace them they could be faulty Make sure all leads that are attached to your dvd\cd floppy drives have secure connections and are not faulty or just replace them they could be faulty. Computers need power and data to travel through every working device to continue its cycle and have an end so any faulty leads will end up with a computer error. Even something as small as a faulty electrical or fan lead can cause you problems
Motherboard and a Hard Drive any leads between them will fail before your motherboard or your hard drive if its a flat ribbon 40 pin type IDE replace it this will be the first to fail. Check all electrical power input and extensions make sure they are securely seated even the cd/dvd floppy drives need to have current go through make sure these drives are working also check the Cmos battery and computer RAM modules make sure they are securely seated with no dust built up or in the sockets on some motherboards cmos batteries are soldered in. Check you CPU central processing unit make sure its securely seated and has thermal paste it might be getting to hot the thermal paste will help disperse the heat Thermal compound is a sticky paste that is placed directly onto the CPU.
http://www.hardwaresecrets.com/article/Thermal-Compound-Roundup-February-2012/1490 Allowing for a more direct heat transfer between the CPU and HEAT SINK and preventing air gaps from forming between the CPU and heat sink. Another possible reason could be a memory dump you might be running to many programs at the one time putting to much strain on the CPU central processing unit. If you can get your computer stable enough scroll to bottom toolbar bottom of screen right click select task manager applications you will see whats running if you see anything you dont need running select end now also under processes see what is ticking over again if you see something you don't need to be running end now or you might uninstall that program and some unnecessary programs not having enough computer ram can cause memory dumps If you are not sure if a module is right for your system use the
Crucial Memory Advisor tool for a list of guaranteed compatible modules. first you will need to select the manufacturer from a drop down list then select your product line from a drop down list then select the model then click find it will take you directly to compatible ram also you will have an option to scan your system you should always for a guarantee on any ram modules when purchasing them or buy working pairs if possible. You might have a virus or malware. Download at least one of these of course it will depend on your operating system.
http://www.majorgeeks.com/Microsoft_Security_Essentials_for_Windows_d6242.html microsoft security essentials your operating system must be registered Win7/Vista/XP 64 bit
http://majorgeeks.com/downloadget.php?id=4281&file=1&evp=dbb3b0aebe6a6a4ff18089a6489a5e62 anti malware free edition Win XP/2003/Vista/Windows7
http://windows.microsoft.com/en-US/windows/products/security-essentials/product-information malware protection
http://100-downloads.com/download.php?p=615 windows xp microsoft security essentials
http://100-downloads.com/download.php?p=614 windows 7 microsoft security essentials Hope this helps.