@timroanumati--No, its not a pirate version.

@quicksiler---If the fan is not working properly then should not the CPU shut down too? But in my case, only the monitor goes blank.

probably your monitor is set to sleep after few minutes.

NOPE THATS NOT THE PROBLEM.

Blank Screen After Installing Windows 7: Fix

After
installing Windows 7, you may experience an issue in which the
screen(s) becomes completely blank directly after startup. This problem
primarily affects multiple-monitor setups.

Fix In order to fix this issue, you first need to regain
access to onscreen display. You can do this by either booting in safe
mode (tap the F8 key on your keyboard repeatedly as the computer is
starting up) or by temporarily disconnecting all but one of your
monitors.

After you have started up and have access to your display, you will need to create a .bat file that kills the dwm process at startup. Follow these steps.

1. Launch the Notepad application and create a new text document
2. Type the following text into the document: taskkill /F /IM dwm.exe
3. Save the file as displayfix.bat
4. Copy this file to C:/ProgramData/Microsoft/Windows/Start Menu/Programs/Startup
   
5. Restart

After you restart, Windows Aero may be disabled. To turn Aero back
on, right-click on the desktop and select “Troubleshoot problems with
transparency and other Aero effects” then enable Aero.

source: http://windowsfixup.com/2009/04/blank-screen-after-installing-windows-7-fix/

@Test_Taker--My computer does not go blank directly after startup. It goes blank after using the computer for sometime like 10 minutes or randomly.

@Bhojpure01--No, I have not upgraded the RAM.

Guys; Thanks for all your suggestions. Its actually not my computer, its a friend of mine. Today i am bringing that CPU to fix it. He had told me that its like 5 yrs old computer. Now i am starting to feel that the problem is with the following thing which is the system requirement for windows 7:

So, how do i check for these things?