Windows XP Pro.

First off it updates itself on your system.. i realise this isnt 100% hackproof.. so i still go to microsoft.com to get my stuff.. but its convenient. Also, the pretty graphics can be disabled and if you have a fast system built for 3d.. lets face it.. heh.. the graphics for the layout dont make a shit of a difference. Its the newest OS so there releasing updates for it over anything else. Its userfriendly for the beginner users as well (im thinking on a overall picture here). I also feel the code is quite cleaner in XP and therefore is quite fast. Cya.