Thanks for the replies.
Unfortunately, I'm still getting problems. I've followed your advice wazoox and there is an IIS user account on the web server which I've given read/execute permissions for the executables I want to run (specifically now, psexec.exe). Its still not running it though - its as if it runs half of psexec and then hangs.
I'll confess to being a complete novice when it comes to IIS - anyone know of anything online that I can read up about it?