|I do have Windows XP Service Pack 3 installed ...|
I really hate to be the bearer of a very bad news. QUOTED from a blog posted by the team responsible for the development & release of IE8 RC1:
Windows XP SP3 and IE8 RC1 will become permanent. You will still be able to upgrade to later IE8 builds as they become available, but you won’t be able to uninstall them.
The only way to undo this mess is to format & reinstall XP.
Curious, where did you get IE8 RC1? You won't have been able to download it from M$ site because the option is very OS-specific.