IIS suddenly not working

I have had IIS working fine for months, opening asp/cgi/pl files with localhost. Today, no: files accessed through localhost generate an error message in my browser of "file not found". This is the second time this has happened this week.

When I go into IIS Manager, beside "default website", the status reads "stopped".

Two days ago, when I was having these IIS problems, the status also read "stopped". I uninstalled UltraEdit, a program I had downloaded last week, and then the status read "active" and then IIS worked fine. (Obviously, that solution is not available this time, as UltraEdit has already been uninstalled!)

When I right-click on the default website, then click "start", I get a message: "The address is already in use". So does this mean that the problem is that some other application is using port 80, which is necessary for IIS? If so, then is there a way of finding the culprit?

Please note that when this problem happened two days ago, uninstalling and re-installing IIS did not help at all.

I would be very grateful for your assistance.

Maybe you have a virus or spyware that's taking over port 80. You can use netstat from the command line to see what ports are listening on your computer.