It might be due to Windows 7. As you might know, Inquisit 2.0 is not updated anymore and has not been tested under Windows 7. There have been a few, but unfortunately very unspecific user reports about problems with Inquisit 2.0 on Windows 7. However, your problems may also be due to a number of other factors (listed in no particular order):
(1) Are you running a 32 or a 64 bit Windows 7 edition? In case of 64 bit, there might be a 32bit (Inquisit) vs. 64bit (OS, drivers) incompatibility involved.
(2) Have you verified that the response box itself is fully functional and properly installed (drivers, etc.)? Does it work with other apps?
(3) Can you send / receive TTL signals to / from other equipment via Inquisit? Have you tried using the Parallel Port Monitor tool (Tools -> Parallel Port Monitor)?
(4) Have you double-checked your /inputdevice settings? I.e. are you using the correct port address as reported by Windows Device Manager, etc.?
(5) The Inquisit documentation contains a number of potentially helpful topics re. response box configuration and TTL.
Inquisit 2.0
http://www.millisecond.com/support/docs/v2/html/howto/howtoportmonitor.htm
http://www.millisecond.com/support/docs/v2/html/howto/howtoserialresponsebox.htm
http://www.millisecond.com/support/docs/v2/html/howto/howtosendport.htm
Inquisit 3
http://www.millisecond.com/support/docs/v3/html/howto/cedrus.htm
Hope this helps,
~Dave