Go to eBuyer where you can get CD writers for well under £20. click here. Also you can get DVD writers for a lot less than £30.
A DVD writer will write CDs and DVDs and swapping with your existing drive is straightforward.
You can also fit it alongside your existing drive, but one of the drives needs to be set a a slave and the other as master. This is a simple jumper setting and should be detailed in the instructions on the new drive.
Failing that look for the jumper next to the IDE cable connection and on the case of the drive you should see CS, SL and MA. Make sure one jumper corresponds with MA and the other to SL.