Yes, it *is* OS dependant, as I said on the first line of my reply. However you're wrong about the tick count, at least according to the Win32 manpage:

Win32::GetTickCount()
    [CORE] Returns the number of milliseconds elapsed since the last
    system boot. Resolution is limited to system timer ticks (about 10
+ms
    on WinNT and 55ms on Win9X).
[download]

Its resolution is limited, however it is in miliseconds.