When Windows Xp activation time period expires and you see error message like this

Then to set windows Xp to its previous licensing stage that is for 30 days activation time period, follow this procedure as mentioned below:-

Boot into “Safe Mode”. To do this, turn the laptop on and press F8 as it is booting up. Go into “Safe Mode” by choosing the “Safe Mode” option. Once you are at the desktop in “Safe Mode”, Click “Start” then click “Run”.
At the run command prompt type the following EXACTLY

rundll32.exe syssetup,SetupOobeBnk

Note That: The “Oo”‘s in Oobe are “oh’s” – not “zero’s” There is one space after rundll32.exe. It is case sensitive as far as I know.

Click OK

Wait a few seconds – the screen may blip a few times or so.

This just reset Windows Activation for 30 days.

Reboot the PC into normal mode – log in – re-activate Windows.

If you are getting Windows is not genuine error messages in windows 7 like this one

To remove this annoying message from your computer and to  set windows 7 to its previous genuine state. Run this command on command prompt:

C:\> slmgr /rearm

Then reboot the computer after the commands stop running. you will find there will be no notifications that can bother you now.

But Remember that you have to login as Administrator for this command to run successfully.