I'm citing the important part here, to let me find it more easily:
Believe it or not, Windows does support focus-follows-mouse, though there is no GUI configuration exposing it. Instead you must edit a registry key and then log out and back in for the change to become effective. You can use regedit to edit the key.
you need to change a binary-valued registry key:
This is a little-endian bitmask. For focus-follows-mouse, add the flag
For example, my XP SP2 laptop originally had a value of
9E 3E 05 80, which is
0x80053E9E. To activate focus-follows-mouse I changed to
9F 3E 05 80in regedit.
Helped me a lot on a Vista machine yesterday. Don't forget to log out and back in again after the change.