It's probably due to 'bundle' kits out there.
You should probably take a look at your customer's install history- I generally find several malware apps installed the same day as things like games, video codecs, video players, optimizers, specialty browsers, or other 'helper' apps.
Also, if they are using a P2P app, such as utorrent, then they are automatically at a much higher risk of malware/viruses.
Make sure you have completely cleaned the computer as well- if something isn't caught then it will reassert itself on the system.
EDIT: I hope you're using more than just JRT...a good antivirus combined with a malware app like Malwarebytes makes a pretty formidable system. I used JRT mainly for cleanup after the fact.
Okay, so "bundleware". Makes sense now. Would do the re-infesting. All they have to do is go at it one more time, whether on purpose or by accident. I have suspected that there is a user personality to junkware, as I have some customers that go for years without ever so much as being touched by it. And others that attract it.
I hadn't used PDF Creator for years. Oh Boy, did that turn into bundleware! (I use DoPDF now.) So I can now see where this would all come from.
If the customer will let me, I put Kaspersky on their machine. But, usually, the have time left on their AVG or their Norton and they think I am trying to sell them stuff they don't need. (And, I do have to admit, I do get so tired of that attitude of distrust. We consultants are the ones writing these things so we can get paid to remove them, don't you know.)
I show them Leo LaPort's video of him stomping on a box of Norton (all funny), but that doesn't always do the trick. They just say that when their Norton expires, they will consider it.
They will let me take away their user's admin right though, which makes them more like Linux and OSx. As I said, that does the trick.
I do carry Malware bytes and have used it frequently. But, I have yet to have it catch anything. Maybe I am too good that cleaning things up first.
Thank you for the advice! Bundleware. Dang!