Looks like developing an automated C++ routine to determine existing web sites would involve a little more work than I can muster any time soon. (Background: I use a C++ routine in setup so that the larger VFP runtime won't be required for distributions, just the smaller VFP MT runtime. Unfortunately, this means I can't use VFP for an interface which would make doing this far easier). So, unless someone else wants to tackle this issue, I'll stick to the original routine which has the defaults most web sites have and allows the manual entry of names if they are different.
For those without those defaults or who need to use other domains/IPs/WebSites, you'll need to know those names before setup. Either go to the IIS console and make a note of the name or run the following code from VFP:

IISOBJ = getObject("IIS://LocalHost/W3SVC")
For each Object in IISOBJ
	if (Object.Class = "IIsWebServer") then
		? "WWW Site: " +Object.Name + " - " + Object.ServerComment
	endif
endfor