You will need to argue your point with some evidence or strong suspicion (about whatever your concern is). However, I would start by saying, "Oscar, thank you for your help (etc.). You recommended running an anti-virus scan which has been completed. Although the ad-ware found has been removed, I am very concerned that this may not have been the actual cause of the problem (you'll need to explain why here). My concern is that, if it was not simply a matter of ad-ware we could have a serious security risk on our hands."
You'll need to propose something here: do you have any ideas of what else could be done? Perhaps you can frame your suggestions or your pleas in the form of a question, "Can we try X?" "Would it be possible to do X?"
You could also end by saying, "I respect your opinions and all that you do for us. I just want to make sure we are doing all that we can to protect ourselves." Using "we" and "our" suggests that you are a team and you are not trying to be be confrontational.
Another way to do this would be to speak with him in person and say, "Here's my concern..." In-person conversations might be better to avoid misunderstanding or an email seeming harsh.
