IIS logs are residing under c:\windows\system32\logfiles by default. (on windows 2003 server)
It should be named like W3SVCxxx
I have an internal website, and its used by SharePoint, so I dont really need to store IIS logs older than 1 week. There is no way to automatically delete this logs. They can grow up to 50 gb in 1 year!
Its safe to delete them, just keep the recent.