To head potential problems, please install this hotfix from Microsoft before installing SP3, especially if you are using a Creation Station.
http://www.microsoft.com/downloads/details.aspx?FamilyId=230796A7-54D6-4C31-BDA3-EFD2F7E87A8C&displaylang=en

If SP3 caused you an issue with your system rebooting or other issues, you may need to follow one of the steps listed here, most likely 4: http://support.microsoft.com/kb/950249/en-us