I've done this before, but don't remember where it was (just that it wasn't obvious).

At work, I have my laptop on my desk with an additonal (larger) LCD panel connected to it. The LCD is to the left of the laptop, but with an extended desktop, the default is to the right. So when my mouse goes off of the right side of my laptop monitor, it enters on the left of my LCD (backwards). This is OK, when the extra monitor is to the right of the laptop, but goofy when it's to the left. (this is kinds hard to explain)

Where do I reset this? the obvious place (to me) would be in desktop\properties\settings tab where you set the Extended Desktop. I don't want to change the "Primary Monitor".