How to detect when a user switches between windows (in WinXP)?

[mangoldproject at gmail.com]

Hi everyone,

I'm a beginning-to-intermediate Python programmer with some experience
in other languages. At the moment I am trying to write a Python
program that will run in the background and execute a series of
commands whenever I switch between windows (my OS is Windows XP). For
example, I want my program to do something when I switch to my Firefox
browser, and then do something else when I switch to a certain sub-
window in Photoshop (which has a certain title, and a certain photo
associated with it), then do yet another thing when I switch to
another sub-window (with a different title and photo) within
Photoshop.
The particular actions will be based on the window's title. I've
already figured out how to retrieve that, using
GetWindowText(GetForegroundWindow()) from the win32gui module.
My question is, how can I detect when the user switches between
windows? I haven't been able to figure that part out yet.
Searching the group didn't give me any answers.
Your help is greatly appreciated. Thanks!

Assaf.