One day you boot up your old trusty Windows XP PC and it tells you "This copy of Windows must be activated before you can log on." You can't boot into Normal Mode. Even Safe Mode with Networking doesn't work.
Here's how you can get back into your Windows in Normal Mode.
1. Boot into Safe Mode (without Networking, or it won't work).
2. Run the following command, rundll32.exe syssetup,SetupOobeBnk.
You should be able to boot into Normal Mode now.
Note that your Windows XP activation has now reset to 30 days so, re-activate Windows as soon as you are able to avoid interruptions.