At work we have a Core 2 demo running Windows Vista. This demo is not connected to the network because we don't want people coming in and changing the background image to display something obscene or pornographic. There is a neat little trick that I found one day while trying to figure out how I could reset the time until the user had to activate Windows Vista.
What to Do
When you log into Windows and your grace period has expired, you can still use Windows with reduced functionality. In reduced functionality mode, you are still able to use IE7. This is where Microsoft dropped the ball.
Click on on 'Access your computer with reduced functionality.'
You can still use IE7, from here just go to File->Open.
Once the dialog box is open, you can then browse to the C:\Windows\System32\
directory. In the file name field, type *.exe
and look for cmd.
Right click on the cmd and go down to Run as Administrator. Allow Windows to run cmd with the administrative token.
At the command line, type slmgr -rearm.
The system will tell you that it successfully executed the program. Now you can reboot your machine and log on with another 30 days to activate Windows.