My HD disk is full, does it help if I temporarily move 5 GB to the garbage can ?? - Some applications like Word won´t save a document if I don´t do anything about the "overweight" in the HD.
Incidentally, OS X is likely to become unstable without some HD space left to run virtual memory etc in. You should aim to keep at least 5GB or so just for normal running.
If you can't archive anything off to CD/DVD, then you might want to considering picking up an external drive that you can store things on - it's also useful as a backup.