This will return the whole name of the OS.
Code:
//echo -a -> $wmiget(Win32_OperatingSystem).Caption

This will only return "7"
Code:
//echo -a -> $gettok($wmiget(Win32_OperatingSystem).Caption,3,32)

I dont have windows 8 installed, but my guess would be that the name would be the same but a 8 instead of a 7.

moo.dll is outdated and i dont think it work so well with the latest version of mirc.