If possible, I will. (It probably means "no", but politely.)

"I certainly will [do it]" means that you will do whatever you can to make "it" happen, "it" typically a a meeting, event, project, etc. "if I can" means that you are unsure whether you will be able to do "it". You may not get the chance to do anything. If you could, you would, but if you can't, you won't! Therefore, you can see why, as Randy says, "I certainly will if I can" equates to a probable no. It means "Don't get your hopes up, but it may happen" and allows you to show interest without declining or being rude or something. However, you shouldn't say this unless there's a real possibility, because it can come back to bite you =P Hope that helps.