Don't know if you need something more fine grained than this but if your OS is NT 4, Service Pack (any) then you are running IIS4. If your OS is Win2K then you are running IIS 5.
While there are upgrades and service packs withing a release, I don't recall any particular scheme in IIS for subversions.
If you still have some IIS 3 around, then I am sorry.