Gratzie to you and your shop.
Well, I don't handle any commercial computer shop, but I handle virtually the security of my lab group's computers (6 units). We had a problem before of computer tampering (around 2 years ago...which we traced backed from a chinese graduate student) thus from there onwards, I was trained to handle security issues of our workstations (even to the point of attending RSA Conferences if we had the time).
I suggest always use legit softwares (especially the OS)... Windows security patches (that usually comes every second Tuesday of each month) are your best friend.
However, take a look also on Microsoft SteadyState, it has the advantage of being...well... free
Best of luck to the new compu shop of the treadstarter...