Hi,
I am developing Inventory control system for couple of years. After gaining user requirement and also selling our product in these period, I have kept adding new features to the application without proper manner.

At this moment, I have about 5 - 10 different major "versions". Each "version" is maintain seperated. IOW, if any bug found, I have to modify at least five copies of code.
Again, I also faced problem that, different client may need different features. That is why I have so many versions now.

I would like to get your suggestion that, how could I prevent this? How to maintain couple of versions simultanously, which no need to modify the same piece of code so many time, and also can deliver the application to client base on version?

Does OOP take place on this?
Does Visual Source Safe is where to go? any other tools?

Any other suggestions?

Thank you