I don't know if this happens with any other OS besides Vista, but it's never happened to me in XP.

I have a Dell Inspiron 1521 laptop. Most of the time I have it hooked up to a large monitor. Well, being a college student, I take my computer around campus sometimes. So I keep switching between the laptop monitor and the monitor in my dorm. Sometimes when I change the way things are displayed, like connecting/disconnecting the monitor, a strange thing happens. The desktop suddenly zooms all the way in until the icons are so huge that only a few will fit on the screen. Then, when I go into Internet Explorer or Firefox or anything like that, the page will automatically zoom all the way out. If I try to fight the zooming by zooming back, it will fight back. The crazy zooming makes it impossible to use anything without losing my sanity. The only way to get it to stop is by restarting the computer.
Every forum, faq, and web site that I have searched does not have even one single person that this has happened to. Not even Microsoft's support site!

Why does this zooming thing happen and what can I do to prevent it from happening?

try..
right click on ur desktop
then personalize for vista or properties for xp
and then go to display settings
and play around with it until the screen isnt zoomed in anymore.
it has happened to me and i did this and it stopped.