Ive got this problem too. This isnt the first time, I also had it on a different computer, same version of pro tools (7.3) same operating system (xp).
I had solved it but cant remember how, I still have pro tools on the other computer working fine. Ive compared the midi drivers, the none working one uses diomidi.dll under legecy audio drivers which isnt working properly, the working one has digidesign midi drivers (which I dont understand as they are the same version of pro tools??).
I found this article
http://www.digidesign.com/index.cfm?...1&itemid=15394, which tells you how to remove these driver, but has no information why and how to replace with the Digidesign Mbox 2 drivers.