There are at least 2 ways to find out which version of Windows you are using.
Right click on “My Computer” | Properties. The General tab will show the information.
Open a command prompt (Start | Run. Type cmd and press <Enter>). Type winver and press <Enter>.
You can put an annunciator in a corner of your desktop saying which version of Windows you are currently running. Microsoft's free, unsupported tool TweakUI XP on its General tab has a box you can tick; your version and build of Windows will display subtly in the bottom right-hand corner of your desktop.