Windows 7 - Win7 activation

So, I installed Windows 7 on the laptop of a friend of mine, but forgot to activate it in Control Panel > System, and now the 30 days have passed. When I put the activation key that's on the CD box, Windows 7 won't recognize it.

What to do?

Do a Repair Install which resets activation.

OK, I'll do it, thanks. You don't lose any data and/or files, right?

Hi,
Reset the 30 day time period for activation by opening the command prompt as administrator. (Right click the command prompt icon and select "Run as administrator")
Then type
slmgr -rearm
Hit enter
Wait for message that it was succesful, then

Reboot and you should have 30 more days.

I would try the slmgr command given first to see if it will work, as the Repair Install is always there as a last resort to reset activation. It keeps all programs, files and settings in place while reinstalling the OS.

Here are the full set of slmgr commands which can be useful, same for Windows 7 as Vista: http://www.mydigitallife.info/2007/03/04/slmgr-commands-and-options-with-windows-vista-product-key-activation/

I tried the slmgr -rearm function, and after reboot the system said I had 3 days left till activation. But when I tried to activate online, it said the license key wasn't valid or something, again. I really don't get it.

What exactly does the message say?

Where did you get your copy of Windows?

Use activation phone option to see if they can get it activated easily.

If not, call MS Customer Service and they will verify key, then open a Support case to get it activated. This also documents ownership and protects you against any further reactivation problems.

Can't say for sure as I don't have the computer with me right now.

I bought my copy of Windows 7 on Amazon. I installed it on my laptop and activated it easily. But on the laptop of this friend of mine, I forgot to activate it inside the 30 days window, and now I can't seem to anymore. Maybe I'll try the Repair Install thingy.

You know that you can only activate on one machine at a time, right?