I found it very useful to know on what version of Windows the installer is running on.
Therefore I made a small patch that implements this functionality.
In you installation script you can do something like this:
WindowsVersion Win32 WinNT
ExecWait '"$WINDIR\notepad.exe" "$INSTDIR\Warning.txt"
The first jumptarget is taken if it's Windows 9x/ME, the second if it's WinNT/Win2000
If you're interested in this patch, please write me. I will send you the patch or the final executable.