Debug DLL error - reverse engineer perhaps

I have a fully licensed app with DLL's etc. It used to work fine, certainly on XP and I'm pretty sure on Vista. However I have tried to install is and run on Windows 7 and one of the DLL's is throwing an error. It is supposed to connect to the webcam but is returning an error code -4 and sometimes -3. There is nothing in the Docs I have to explain why. The company is no longer around to ask. These DLL's are important to a project of mine and this part of the project now does not run. I am hoping you could ; a) find where in the DLL it is raising the error b) hopefully be able to tell me Why that error is being raised so I can try and find a solution, or hopefully you might even suggest a solution. I just had to enter a figure in the price field below so I just entered something, you can tell me what you think it will cost. You must be experienced in reverse engineering. thank you