to address bugs:
[ 555812 ] installing extension w/o admin rights
[ 555810 ] removing extensions without admin rights

* When enumerating the Python versions found, also remember the HKEY
  they were found under.
* When installing, if Python was installed under HKCU, we will too.
  If Python was installed under HKLM, we check the permissions of
  the current user, and install where we can.
* The "root" key we use is a global variable - all registry setting and
  delete functions use this global rather than a hardcoded HKLM.
* A new entry is written to the install log, indicating the key we used.
  Uninstallation is based on this key.
* 'tempnam()' is used rather than 'tmpnam()' - 'tmpnam' creates a temp
  file on the root of the current drive, and if this is readonly would
  explain the 'freopen' errors occasionally reported. 'tempnam'
  creates the temp file in the %TEMP% directory.