If you have SP1 installed, your Help | About screen will have the following first line of text:
Microsoft=AE Office InfoPath=84 2003 (11.6357.6360) SP1
You will need to remove the InfoPath SP1 Preview before installing SP1, and then reinstall InfoPath after that.
For more information, there have been threads on this on the release date of SP1.

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Greg Collins [InfoPath MVP]
Please visit: http://www.InfoPathDev.com
Is InfoPath 2003 11.5531.5703 a preview version that should be removed before installing Office 2003 Service Pack1