No sooner had Microsoft released the second beta of Internet Explorer 8, then it warned users of Windows XP Service Pack 3 (SP3) might not be able to uninstall either the service pack or IE8 in some circumstances.
This is not unusual. When Microsoft released SP3 last May it warned users that they wouldn't be able to downgrade from IE7 to IE6 without uninstalling the service pack.
More;
http://www.internetnews.com/software/article.php/3768401/Microsoft+Issues+Warnings+on+IE+8+Beta+2.htm