works in visual studio but not when deployed to web server?

Hello,
I have a simple one page app that, in it's current 'testing' version, simply runs the ping.exe command (on the server), captures the output and displays on the web page. It uses windows integrated auth (intranet), but the process (ping.exe) is run as a different user, set by setting the relevant properties on the system.diagnostics.ProcessStartInfo object that is passed the system.diagnostics.Process.Start() method. This all works fine when run in VS, but fails when I try to run in from the target web server/site.
There is no exception... the process exit code, which I'm showing on the web page, is -1073741502. That is when run from an IIS 7.5 server, when I run the same code from an IIS 6 server, again, no exception, but a system error dialog appears saying something about the process failing to initialize. In both of these cases I'm accessing the page from the web server itself, which is w ...