Also, on a win98 machine, there is no such OS Environment variable.
...which means that if there is no variable present, you know that you're on 95 or 98. (Not sure if Windows ME has it or not)
other differences: windows 2000 uses a default installation folder of c:\winnt, while XP uses c:\windows
you can tell 2000 and NT4 apart because 2000 also has a variable named 'appdata" which NT4 does not.