If you running the beta version of Internet Explorer 8 (IE8) under Windows XP, the word is to uninstall IE8 before installing Windows XP Service Pack 3 (SP3). Once SP3 has been installed, you will be unable to remove IE8 (and IE7). The reason for this is documented on the IEBlog with this post. Basically, when you install IE7 or IE8, the existing IE6 (version of IE that comes with XP) is backed up to an uninstall folder. SP3 includes an updated set pf IE6 files. If you were to uninstall IE7 or IE8 after installing SP3, you would have a mixture of IE6 files from pre-SP3 and the bits installed as part of SP3. That would cause all sorts of problems and Microsoft disables the uninstall option for IE7 and IE8 after SP3 is installed.
Not being able to remove IE7 isn't a big issue. If you have IE7 in place, the odds are that you are used to it and will not be removing it. The beta version of IE8 is another situation altogether. When the next beta or release candidate or even the actual release comes out, you'll want to uninstall the beta version first. It's just the prudent thing to do. Well not exactly, it's much safer to install anything labeled beta in a virtual machine. But if you have the IE8 beta installed on a XP box and you are planning on installing SP3, you'll want to remove the beta before installing SP3. In fact, Microsoft will not offer SP3 through Windows Update if it detects the IE8 beta on your machine. Once SP3 has been installed, you can install the IE8 beta again.