Service pack update failed

Updating a W2K pc to sp3 in order to apply the KB823980 patch, received the error msg:

“Setup has detected that the version of the Service Pack installed on your system is lower than what is necessary to apply this hotfix.

At minimum, you must have Service Pack 2 installed.”

I know Service Pack 3 was successfully installed. What are my options here? I know I must get the patch installed.