windows xp home edition won't load

I bought a used HP xt963 computer with windows XP home edition already on it. However, when i boot it up i get the following message screen:
Windows Product Activation
This copy of windows must be activated with microsoft before you logon.
Now, my question is just how the HELL do you do this when it will only allow you to shut it off and you can not reach any of the programs or even the desktop?
Jimni02

 

this is the 1 800 number to microsoft 1-800-936-5700 ...worked for my friend