Compress Old Files help-

Someone posted that it was a good idea to run Disk Cleanup- I always do that prior to doing a defrag weekly. But I have discovered that although I have the box checked to *compress old files*, the figures never change. There are presently 77,397kb in that column, and it never gets wiped out- Any help here? All the others go to 0 after the disk cleanup. I use WindowsXP with all the updates.