On my Apple USB keyboard, I found that the zero key on the keypad (that is, the wide 0 key to the left of the period key) toggles insert mode in Windows XP.
If pressing this key generates a zero instead, try toggling Num Lock. On my keyboard, the "clear" button (just above the 7 and just below the reduce-volume button) functions as Num Lock.
Incidentally, for a while I was stymied in figuring this one out by Notepad. Turns out that Notepad doesn't obey the insert mode! Or at least it didn't for me. But the zero key successfully functions as the insert toggle for me in both Word 2003 and the Windows command prompt.
