It's obvious that one of the tasks a computer maintenance guy is going to be asked to do is to deal with virus/trojan and other malware infections and to get rid of them. It wasn't too bad in the early days of computers when viruses were more of an annoyance rather than harmful but todays viruses and trojans are much more sinister, looking for banking and credit card info and suchlike. They are also much more difficult to get rid of and have even become 'intelligent' in that they will actively hide themselves away and even prevent downloading and/or running of removal tools.
Given that, can we ever be 100% certain, beyond a shadow of a doubt that we have absolutely, categorically got rid of the nasties? It's one thing to try and get rid of nasties on my own machine when it's my own bank account/credit card details at risk but when it's a paying customer and their details at risk...?
A mate of mine doesn't mess about - at the first hint of an infection he deletes the system partition of his hard drive, creates a new partition and then rebuilds from a known good, clean image that he's got. Is that overkill or, considering that a paying customer's details are at risk and I'll be putting my name to the repair, is it the best thing to do? Any other suggestions on disinfecting a machine and knowing that it's 100% clean and safe after the procedure?