Ok thats probably your problem. J
You will have to remove the DVD drive once it is removed look at the back it will look like this,

I have indicated where the jumpers are located. The jumpers tell the computer how to read the drive (every IDE drive hard drive, cd/dvd rom, floppy drive has them) the options are Master (meaning it is the main drive), Slave (secondary drive), and Cable Select (the cable decides how the drive will be read).
There are two ways to connect the drive to the motherboard one is to split the IDE Cable with the hard drive the other is the drive has its own IDE Cable and plugs in next to the hard drive.
If you are sharing the cable with the hard drive configure the jumper as Cable Select,
If your drive connects with its own cable select Master.
To do this pull out the jumper and insert it in the location you want.