James,
Depends on the very of IIS...
Windows 7 AND Server 2012 AND Server 2008 R2
IIS 6.1 & 6.2 all you do is click on "Sites" under your machine name and it will list in the right hand window the Site Name, ID, Status, Binding, Path.
Windows Server 2003 R2
IIS 6.0 all you do is expand "Web Sites" under your machine name, right click "Default Web Site" and go to properties. In the new dialog that pops up choose the Home Directory tab and look for Local Path