You can set the log file and format in the IIS manager. Bear in mind that some versions of IIS don't log immediately but buffer logs. Some versions of IIS also leave a lot of nulls on the end of the file.
I'm not sure whose IP number and username you're trying to get. If it's the end-user, you cannot retrieve their username unless they're using HTTP authentication.
WIth IIS you can log the IP address of the client (indeed, normally you do).
Mark