If you haven't installed anything new since sp3 and need to get the system working asap, you could try booting in safe mode by tapping f8 key while your computer is booting from an OFF, or restart condition; when the boot menu pops up, select safemode. Once safe mode boots, open the control panel then add-remove programs, if sp3 is not listed in the add-remove programs list, try clicking the option to show security updates? then find and uninstall sp3, it will roll your computer's OS back to how it was before installing sp3. Here is a link to support.microsoft.com's article on how to uninstal sp3 http://support.microsoft.com/kb/950249/en-us