This just goes to show that a little bit of effort can go a long way in keeping the customer happy. You are right, his work may not have made a direct sale but I am sure you will strongly consider Evesham next time you buy a PC as a result. Word of mouth advertising is not only the most effective but also the cheapest. There will be many readers of this site for which comments like this will swing the arguement between an Evesham PC and that of another company.
As it has been said, good customer service cost nothing but earns volumes.