taken from tom's hardware :
the rest of the article:
http://www.tomshardware.com/news/Windows-XP-SP3,5334.html
The problem relates to machines with SP3 installed rebooting and then not allowing users to get so much as Safe Mode. The problem is mainly affecting AMD users who bought their machines from HP. Johansson attributes the problem to the way that HP puts images of Windows onto its machines, using the same copy for both Intel and AMD boxes.
the rest of the article:
http://www.tomshardware.com/news/Windows-XP-SP3,5334.html