|You can only download it from Microsoft.|
If it messes up your Windows, there must be some pre-existing problem with your Windows installation.
You must disable antivirus and antispyware software before starting the SP3 installation. First do a full virus and spyware scan, remove any infections found, then disable antivirus & antispyware.
If your previous attempt was by "online" installation of SP3 from Windows Update, try downloading the complete SP3 package for offline installation, then try installing that.
You can get it here: http://www.microsoft.com/download/e...
Don't forget to re-enable your antivirus & antispyware when installation of SP3 has finished.