I was using an older version (71123 instead of 71223 - I was fooled by the
very similar version numbers).  I uninstalled 71123 but am unable to install
71223.  I'm getting the same "error 2909" from the installer that I got before.
Last time, removing the whole "ISE" key and subkeys solved the problem but this
time it did not.  So I can't go any further on this on Windows XP at the moment.