If you're referring only to the directories entitled WINDOWS, WINDOWS1, WINDOWS.old, etc, there aren't any 'important' files worth keeping that your latest Windows directory doesn't have already, or will accumulate in the future. So yes, retain the latest one & delete the others.
However, you may want to retain all your previous 'Documents and Settings' folders till you have the time to sift through their sub-folders to see if you have any data (including emails) stored there.
And you will need to reinstall all the programs that you had previously. These are stored in the 'Programs Files' folder, so don't delete this either. G