That link no longer works. Try this one instead click here.
I forgot to respond to your question about why these are left in the registry. Some programs like the infamous Kazaa actually deliberately leaves some programs installed [usually known as Spyware], others bypass the registry if other bits of the program are left and some just don't bother - presumably in case you decide to reload them. Then there are the timed programs on a trial basis that also leave their mark on the registry to stop you simply reloading the program & getting another XX days free to sample the goods.
Regular sweeping of the registry is good house keeping simply to remove invalid entries and dead ends & Regscrub XP is probably one of the better ones to use!