It's probably not gremlins. It may be that there is some fancy mouse software got installed. One program I know about, called Dmouse, you only have to press Ctrl + F12 to swap the buttons over. Quite a few others use this key combination.
My Genius Webscroll mouse came with a software and driver disk and I installed the software, but I pretty soon took it off again. Various hot keys were set up to do things like change the function of the middle button, swap left & right buttons, etc. There were lots of "handy" features, which soon proved to be a pain in the fundament.
If you look in Add/Remove Programs, you may find a similar Logitech utility program has been installed. It seems to be called Logitech SetPoint. According to some reports I have read, if your sister does have that software, she got off lightly!
've been using MS mice for the last ten years, and I'm amazed at how a really great mouse as the Logitech MX 400 comes with such a piece of crap as SetPoint.
"Universal scroll" hides middle button clicks from Firefox, "Autoscroll" is plain horrible in certain programs, middle button has to be pressed two times in Firefox to work, no per application assigments, lack of creative functions...
Isn't there an alternative to this garbage?