my question is it possible to reuse a cdr disc,as i have lots of them some have only 20mb on them,and cannot be written to,they usually get thrown away,is there a way to reformat them and use them,any ideas,

When you write to a cd-r you write one "session".
If you "close" the disc no more can be written to it.
If you leave the disc "open" then you can write further "sessions" up to the maximum capacity of your disc.
All this will be explained in the help files of your burning software, probably in Windows Help and definitely in a million locations using Google.
You cannot Format a cd-r but you can Format a cd-rw. (rw = Re-Writable).
This question has been asked before. Search the forums for more info.

thanks for the tips.