A lot depends on the skill and ethics of the technician. Some will do a proper job, remove the virus as requested and do a complete system scan to check for any more viruses. Others will do Business Software the minimum amount of work and only remove the virus that the customer has noticed, maybe install an ineffective (free licence?) antivirus program and not bother to carry out a full system scan. A good technician will have an effective antiirus program that he knows and trusts. He will not install this on the customer's PC as his licence will not allow it, but he can remove the hard disk from the customer's PC and connect it to his notebook PC with a USB or similar adapter and carry out a scan that way.