The new Windows installer always places the uninstallation registry key into HKCU of the executing user. This is correct for a per-user installation, but when run with InstallAllUsers=1, the key should go into HKLM instead.

The "Programs and Features" list of installed applications is assembled from the HKLM and HKCU keys. In many cases, a system-wide installation will be performed by temporarily elevating the installer to a user account with administrator privileges on the local system. However, since Vista, the "Programs and Features" list is always run in the logged-on user's Explorer process, even when started by a different user (such as in an elevated command prompt). With the uninstallation key going into HKCU of some administrative account, the Python installation will not appear in this list, hence, cannot easily be removed.

Ceterum censeo: This bug would have been avoided by using MSI as the distribution package format.