[R] shell.exec() on Windows, unexpected behavior

[Richard M. Heiberger]

I have Windows XP machines with different software configurations.
My laptop has Adobe Reader 8 and Adobe Acrobat 6.
My desktop has Adobe Reader 8 and does not have Adobe Acrobat.

On both machines, PDF extensions are registered to Adobe Reader 8.0,
and double-clicking a pdf file in Windows Explorer opens Adobe Reader
8.0.

In R-2.5.0, with all Adobe windows closed, the command
  shell.exec('c:/Program Files/R/R-2.5.0/doc/manual/R-admin.pdf')
opens an Adobe window.
On the laptop, it opens Adobe Acrobat 6.0.
On the desktop, it opens Adobe Reader 8.0.

When an Adobe window is aleady open, shell.exec() uses the already
open Adobe window.

Neither machine has any Adobe directories in the shell("PATH").

I am puzzled by shell.exec() opening "Adobe Acrobat" when the file
extension is registered to "Acrobat Reader".  The documentation in
?shell.exec says "Opens the specified file or URL using the
application specified in the Windows file associations."


thanks
Rich