It seems from the information you give, that you may be a bit confused by the differnce in the various media you can write and overwrite.
Standard cd r's = Write once (and if free space still available add to)
cdrw = write, re-format, write again.
Many free programmes exist that may be of help, but I will post this one as it will work for XP and Vista:
It's free, and the help file will explain in detail.