First I copied history.dat to a safe place. Then I used TOOLS - PRIVACY - CLEAR DATA - CLEAR AUTOMATICALLY - HISTORY. I put check in box for CLEAR ON SHUTDOWN. I closed KM and and looked at history.dat in profile. It was the same size as the backup copy. I opened KM and closed again - no change in file size. I opened KM and could see history all the way to last week.
Second method was to use TOOLS - PRIVACY - CLEAR DATA - CLEAR HISTORY. I then closed and reopened KM and I still have all my history.
I also tried clear on startup with same results.
I am on XP-SP3 and the profile is with the program. I do have write authority to the profile.
More info in http://kmeleon.sourceforge.net/forum/read.php?3,104397