Windows 7 just de activated :S

[The Professor]

Tbh just get windows easy transfer to grab your stuff and find a pre-cracked version of 7, I'm sure the fresh install will do you good anyway!

[Casio]

As stated above, try and install a pre-cracked fresh version, I had the same problem as you and redone it with a pre-cracked version and worked, Good Luck!

[Stephen]

The reason why it deactivated is probably because you installed the windows update that does it. I've just been notified to install the update too:

Update for Windows 7 (KB971033)

Download size: 1.2 MB

Update type: Important

This update to Windows Activation Technologies detects activation exploits and tampering to key Windows system files. These exploits try to bypass regular Windows activation and are sometimes included within counterfeit copies of Windows.

More information:
http://www.microsoft.com/genuine

Help and Support:
http://go.microsoft.com/fwlink/?LinkId=161784

Don't set windows to install updates automatically if that's what happened