Start by eliminating 1 potential problem at a time. The most common in your case are 1) RAM and 2) the HDD.
It's not the disc, however you will want to rule this out. The first thign you should do is try another Windows CD. You will want to rule that out first.
Replace the RAM with a different brand of RAM. Even a smaller amount....it doesn't matter.
If you are still coming up with problems, try another SATA drive. You can pick up a test 40g WD SATA (WD400 I believe) for around $30-$40. If you are still having the same problems move to the video card and then the board.