I was working on a friend's system last night I was having problems activating it.
First of all the Windows Activation icon was totally missing, and second it never would pop up and ask me if I wanted to activate windows. If I looked into the event log I could see errors that windows needed to be activated.
The history of the machine is somewhat cloudy. It stopped booting about a week ago, so my friend ran a Windows XP repair from the boot disk. Unfortunately he used an old Windows XP SP1 disk. This had the unfortunate effect of creating a Frankenstein version of windows with DLLs from SP1 and SP2 mixed around....so this probably had a lot to do with why the activation screen was not showing.
After doing some google searches I found a way to force the Windows Activation dialog to show. This can be done by calling this from the command line:
%systemroot%\system32\oobe\msoobe.exe /aUnfortunately when I call this Windows came back and told me it was already activated. This was puzzling. I was getting errors running automatic updates (Saying I was not activated), but Windows Activation still thought everything was great.

I thought that if I could just get windows to run updates...I could get the system back to normal
After doing some more searching I found a way to reset Windows Activation. To un-activate windows you simply...