"The problem is a registry value, present on images created w/ Intel processors, that causes a driver (intelppm.sys) to load at boot. When intelppm.sys attempts to load on an AMD-based system upon the install of SP3, it causes a blue screen and the continuous reboot."Although the Update Product Team reports that a filter will be added to block SP3 from affected systems and are investigating a fix (See Edit Note below), Jesper has a complete explanation of the problem as well as a tool he has created to easily repair affected computers. You can find it at "Does your AMD-based computer boot after installing XP SP3?".
Please note that SP3 is not on Automatic Updates yet. It is, however, available from Windows Update. Also remember that Microsoft is providing free, unlimited installation and compatibility support for Windows XP Service Pack 3 (SP3) through April 14, 2009. Additional information is available at http://support.microsoft.com/oas/default.aspx?ln=en-us&prid=11273&gprid=522131
Edit Note 12 June 2008:
See *Update for Windows XP (KB953356)*
Deployment: Windows Update, Microsoft Update, Automatic Updates, WSUS, and Catalog
Classification: High Priority, Non-Security
Target platforms: Windows XP
Approximate file sizes: ~ 509KB
Description:Install this update to resolve an issue in which your computer mayhttp://support.microsoft.com/kb/894199
restart continuously after you upgrade to Windows XP Service Pack 3 on
systems with non-Intel processors. After you install this item, you may
have to restart your computer.