Windows 7 activation period

After the activation grace period, you enter limited functionality mode. This typically means:

1. 20 Second wait when starting Windows, with a screen saying activate now.
2. Solid Black desktop wallpaper
3. Nag screens popping up at random
4. Notification popup at random
5. You will still be able to do most Windows update, 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.

Also, you can run slmgr -rearm (From the command prompt) a few times in order to extend the deadline by 30 days.