Something like this will happen:
1.20 Second wait when starting Windows, with a screen saying activate now.
2.Solid Black desktop wallpaper with a notification at the bottom right corner about non-genuine copy.
3.Nag screens popping up at random.
4.Notification popups at random.
5.You will still be able to do most Windows updates, but anything through Windows Genuine Advantage will fail, this includes certain programs (e.g. Microsoft Security Essentials) and features such as many downloads from Microsoft.com that require validation.