The service pack is the same if it is AutoCAD you are working with:
Make sure you downloaded the 32 or 64 bit version (the one you have) as installing the incorrect version could also trigger the message.
In the About dialog, see if it says AutoCAD only or AutoCAD Mechanical, for example.
I tried to install both 64 and 32 bit versions of the SP and got the same error for each. I am using vanilla AutoCad and not Mechanical (i have both), but the folder containing my acad.exe file is named AutoCAD Mechanical 2012. I downloaded and installed the Mechanical SP and it fixed the problem.
Access a broad range of knowledge to help get the most out of your products and services.