Failure of IIS service to start

I have a Windows 2003 server that I have been using for serveral months and running Internet Information Services without a problem. After installing an application that creates its own virtual directory in IIS, IIS has now shutdown and will not start under the SYSTEM account. The message I get when trying to start the service under the system account is: “Access Denied”. I have tried uninstalling the program that caused it and reinstalling IIS and did not fix it. Can anyone give me a clue as to where the problem is?