Trying to execute an external program from an IIS script (eg PHP or ASP), but the program behaves differently (or not at all) from when you run it manually from the command line.
This could be a security issue. IIS is trying to protect you. To thank it for its help but send it on its way, open Computer Management, right-click Default Web Site (under Internet Information Services) and select Properties. Go to the Home Directory tab. Change Application Protection to 'Low (IIS Process)'.
Bear in mind, that this could allow hackers to do worse things than they might have managed before.