There are 2 articles regarding the prescott chip on microsoft. One about how to recover if you didn't install the patch. The other about where to go to get the patch for this chip.
http://support.microsoft.com/default...b;en-us;885626
http://support.microsoft.com/default...b;en-us;885627
I have tried the method 1 without success. But you can also try uninstalling service pack 2 from the recovery console when you boot from cd.
They have an article for this as well.
http://support.microsoft.com/default...b;en-us;875355