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
Testing and replacing the PC power supply
http://www.youtube.com/watch?v=hbNxhvm0czs
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 target="_blank">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 do not 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
Capacitors look like small metallic canisters and they are found in many different computer components including computer monitors, computer motherboards, video cards, power supplies and a variety of other consumer products.
A bad capacitor can cause a variety of computer problems, but if you can identify the faulty capacitor you may be able to replace it and save your computer.
Capacitor
Capacitors, sometimes also called condensers, are used to store energy in an electric field.
In the context of computing, capacitors are used to block the direct current being circulated around the motherboard.
A typical capacitor should last up to 15 years, but some computer manufacturers use substandard capacitors resulting in shorter lifetimes.
Computer Problems
When a capacitor has gone bad on a computer you may experience a large range of problems. The computer may have trouble booting up, or it may shut down without notice after running for only a short period of time.
The most common problems associated with faulty capacitors are that they cause unexpected computer crashes and general reliability issues ranging from read/write issues to distorted screen images.
Identifying a Bad Capacitor
To identify a bad capacitor you will need to open your computer case and locate the capacitors on your motherboard.
A bad capacitor may exhibit swelling at the top or the base of the capacitor or it may sit at an awkward angle with the motherboard, so compare the various capacitors in your computer in terms of size and placement.
Additionally, a bad capacitor may have a funny smell or it may have a brownish residue leaking from the top or the base.
Prevention
Manufacturers claim that capacitors may go bad because a computer is not receiving enough power from a power supply, because of an overclocked processor or because a computer is operating in an environment with too much heat or humidity.
However, some capacitors prematurely fail due to faulty work on the part of the manufacturer.
If you have a bad capacitor be sure to do research as to the lifespan of other products made by your manufacturer.
Identifying bad capacitors
http://www.youtube.com/watch?v=ngA4k32jLGc
Capacitor replacement
http://www.youtube.com/watch?v=s0Pn2tEjY04
How to check a capacitor
http://www.youtube.com/watch?v=U4tnHA0phcc
Replacing a leaking capacitor
http://www.youtube.com/watch?v=s0Pn2tEjY04
Could be a problem with the Hard Drive or the Hard drives PCB
http://www.onepcbsolution.com/
http://www.youtube.com/watch?v=du0Qd_3wFWM

Most likely a hard drive problem. When you get the flashing cursor, it indicates a lack of an accessible Operating System. But, if the drive was good you would get a boot failure indication. I would try booting with an XP disc and look for the HDD (or try F2 when you boot) to get to the BIOS and look for the drives there.