Cd-r doesn't have re-writing unless you use live burn. Essentially once it's burned it's done. you can enable live-burn On windows vista and newer computers in some way or another. I haven't used it in a while but it's a feature built into windows when you are going to burn a disk. Not sure if you can edit files you have already made though, never experimented with it.
I think all the responses given here have answered my question.....A friend of mine made some 'Drive Image Back up Discs for my old computer on blank CD-R discs so I thought it would be nice to be able to re-use them. Burnt I suppose he has 'burnt' them once and for all ..... When I put them in it said they were 'write protected' full stop!