Why does my Mac hang when I turn it on?

my 24" mac broke down and had it brought to a nearby repair center, the cost estimate was so expensive that it would be better to buy a new one(the board and the lcd had to be replaced as said by the shop). so I looked for an alternative.. i found a guy who serviced IMacs, he was able to run the Mac on windows 7, he just reformatted the hard drive. but everytime he would load the mac OS X, the screen would just show a blue screen, and sometimes the graphics will appear but will stall. he said that the video driver is already weak and about to give so the reason it works on the windows OS is because he can disable the driver using the dos prompt on the windows platform. is there a way to replace the video card on the IMac? he said that it was built in with the board so it can not be replaced as a part, i have to replace the whole board..

hope anyone can help.

thanks..