If IE8 beta was installed before you applied SP3 then it is locked in place as you should have noticed if you had read the EULA on either one, and can't be removed in the normal manner via Control Panel/Add or Remove Programs. However you have two other chances. Either use System Restore to go back to before it was installed or try the following:
To uninstall Internet Explorer 8 in Windows XP or in Windows Server 2003, follow these steps:
Carefully select and then copy the following command:%windir%\ie8\spuninst\spuninst.exe
Click Start, click Run, type Cmd.exe, and then press ENTER.
Right-click inside the Cmd.exe window, and then click Paste to paste the command that you copied in step 1.
Press ENTER to uninstall Internet Explorer 8. When the uninstall program is finished, restart your computer. Then follow the Next steps to verify that your previous version of Internet Explorer is restored or to contact support if you experience any problems.
After you restart your computer, open Internet Explorer. Then click About Internet Explorer on the Help menu. If Internet Explorer 6 or 7 appears in the About Internet Explorer dialog box, then you are finished.