There have been many reports (Google AMD XP "serice pack 3" or visit Microsofts support forum) of problems primarily with AMD based computers after installing XP SP3.  Tom's Hardware http://www.tomshardware.com/news/Windows-XP-SP3,5334.html was the first report of this we found, but Computerworld http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9084418 also posted an article the same day.