Display Shuts Down

There are many possible problems to based on your statements. There are also many reasons why a computer system encounters failure in booting and displaying images.

The best thing you can do is to try first if there is a problem with the computer monitor hardware before digging down to the computer hardware parts. You can do this by checking all the connection cables to and from the computer monitor.

Well I don’t think that gpu draws that much current but what is your PSU rated for and how many years old is it. It sounds like when you draw a lot of power IE WOW the psu drops to unsafe levels causing the MB to drop the discreet graphics card until the computer goes through POST again.
That would be my first guess is that there is not sufficient power to the system provided by the power supply. This could be why it worked before when you have an older PSU the capacitors tend to dry out and hold less of a charge so when you draw close to the units max it may not be able to provide the power anymore.
Another option of thing to look at is has it worked before? Stupid question but we have to ask. Other then that the game settings out be set beyond what the card can handle so the card fails when the game starts. 
If you don’t have another power supply try lowering the systems resolution to 600x800 and seeing if this allows the game to start. It could be a game setting si to much for the 9400gt.
Side note this sounds like something more of a virus then a help “Bought a Registry Program and fixed over 177 registry errors.” * could be an underlying issue.

CHECK FOR RAM

THEN FOR CMOS BATTERY

THEN FOR VGA PORT

 AND FINALLY PROCESSOR