I have been successfully deploying our application using ClickOnce, but
so far, only on XP workstations. I recently tried to deploy it to a
Windows 2000 workstation, running with SP4 and dotnet 2.0 framework.
The little window titled "Launching Application" comes up, then the
"Application Run - Security Warning" window prompts me to run the app
(because it isn't signed yet). I click "Run", and shortly afterwards I
get a window titled "System Update Required", and it contains the
message:

"Unable to install or run this application. This application requires
your system to be updated to Microsoft Windows Operating System version
5.1.2600.00.

Please contact your systems administrator"

I verified that the application doesn't really require Windows XP by
manually installing it and running it locally.

I also ran a quick verification of ClickOnce by creating a very simple,
small Windows forms app, published it, and successfully installed and
ran it on the same Win2K box.

Someone please help! I can't find anything about this error message
anywhere online. Thanks.