I tried to record over it, but that didn't work it says to make it a blank CD first.
Once you fill up a CD-R, that's it. A CD-RW allows you to re-write data and use the disk over and over again.
Keep in mind, that unlike a floppy or Zip disk, you cannot erase just one file from a CD-RW. You must reformat the entire disk to re-use it. The disk makers say you can do this up to 1000 times.
The process for reformatting varies according to your CD burning software, but most programs will have an "Erase Disk" button somewhere.
Windows XP has built-in CD burning. If you put in a CD-RW that you want erased, double-click, or press enter on the CD recording drive, then under "CD Writing Tasks", click "Erase this CD-RW". The CD Writing Wizard will walk you through an easy 2-step process giving you a blank CD-RW to use.
Backing up a hard drive is when CD-RW's really come in handy, because you can reuse them.