I think its a good idea, remember also to give them the advice so the problem doesn't come back again, practice 'safe computing' and advise your customers to do the same, therefore when they come back, you ask the question, was you following the advice I gave you, using a firewall, adware removers, virus checkers etc. If they say no, then well you have that to back up it up, and trust me, they will come back for other problems then, and as you know the very nature of computers, there is always gonna need the people with the know how!!!
Good luck