off hand, you can use a non-platform dependant web stats program like http://gostats.com. If you are looking for a program to run specifically off of IIS, but is free I would say awstats or webalizer dependant how much CPU time and exploit risk can you spare. (make sure everything is secure and configured properly). Note: even though webalizer claims to be 100% MS free, they still have a Win32 exe binary. (top right corner of their homepage). But if you don't necessarily need to run another program on your server, gostats is a good bet.