You say you put a new DVD drive in?
Make sure the jumpers on the back of the drive are set to primary and slave, and that they are in the correct places on the cable. I installed a new hard drive yesterday, and put it as a slave, because I presumed the other one was a master, but it was infact a master as well.
The confuses the hell out of the computer, and it would get to the POST screen, and just freeze. Might be worth trying anyway?
If not, trying resetting the CMOS. There should be a set of three pins on the board somewhere, there should be a jumper on pins 2 and 3. To reset it, just move the jumper to pins 1 and 2 for a couple of seconds (while the computer is off), then move it back to pins 2 and 3. (You'll then have to enter the time and date and stuff again, but it may fix the problem)