"Windows.old", contains the old Windows files, the old programs files and also your old documents and music etc. Not just the folder "Windows".  The only thing that is potential useful is any of your old files. Move them from the subfolder in windows.old  to the new place if you don't have the. If you have all this stuff then just delete the folder and recover your harddrive space.
Last month I reinstalled Vista on my Inspiron 1525, using the disks shipped at the time of purchase. Everything went well as per "Article ID: 339949". During the process, I noted seeing something about "Windows.old" being left on the C: drive, yet being of of no use. Since this "old" folder is ocupying a little over 30GB, I would like to delete it altogether. Would that create a problem (either now, or in the future)? Also, since my Inspiron is pre 4/22/09, is there any way I can create my own shadow copy, for use as a "Factory Settings" restore?  
"Windows.old" after re-installation