You try to start Default Web Site in IIS Manager and it fails with the following error message:
The process cannot access the file because it is being used by another process. (Exception from HRESULT: 0x80070020)
If Skype is running - quit it. Then start the web site! Your Skype was listening on the port 80.
Start the Default Web Site before starting Skype.
Happened to me on Windows7 64bit, IIS7, IIS7.5