You might have insatlled windows many a time but ever wondered how long your windows installation lasted before you thought it had become very slow and you need to reinstall ? Its easy to find and its windows internals which tells you.
Open the command prompt ( type cmd in run prompt )
Then type systeminfo in command window
You would see huge data being displayed. Just scroll up a little and look for Original Installed Date on left.
This will tell your when you actually installed windows. ;). This works in both windows XP and Vista.