The Windows One-Click installers were removed because they were based
on a modified Aleph One source code, and this modified source is not
available, which violates the GPL.
The Aleph One team is working to correct this situation by
1) trying to track down the old modified source, and
2) replacing the installers with ones built directly from the
   available source.
Until one of these permanent solutions makes Windows One-Click
installers available, you can still play Aleph One with just a few
more clicks.
1) Download the latest Aleph One package from
   http://sourceforge.net/projects/marathon
2) Download the content files:
   - Content files for Durandal and Infinity are available at
     http://trilogyrelease.bungie.org/ under the "A1/SDL FILES FOR
     WINDOWS" heading.
   - Other content files are available at http://archives.bungie.org/
     Just make sure to download the versions for A1/SDL.
3) Put the Aleph One files and the game content files in the same
   directory.
4) Play Aleph One.