MPB Wont Boot past apple logo. Pin wheel stops.

I just recently upgraded the memory to 4gb and the hard drive to a 500gb 7200rpm. The computer ran great the last three weeks. Then all of a sudden, the display colors got really grainy, then the computer froze. Now, the computer will not boot past the apple logo with the pin wheel below it. After about 45 sec. the wheels stops and the computer does nothing. Ive tried booting with the old hard drive and the old memory and still nothing. It will not boot with the install disk either.