find out the OS we're in?

i did a search here, didn't find anything, so i'll just ask it

i need to know how to find out the current user's OS from a program.....

if anyone knows the command to do this, it would be greatly appreciated (and yes, i did search msdn.......but their search stinks)

-thanks in advance

-edit-
and just so you all know-
in other words, I am trying to edit the registry from my program, and i've got it so it works perfectly on XP, but when i try and run the program in 98, it just doesn't even create anything.......

so i'm thinking that there must be something i have to do for each OS.....

if anyone knows about this, i'd like some help :)

GetVersionEx