Any sort of "programmatic access" to your CCTV system will be very dependant on your vendor (of either the camera system itself or the additional network.web interface).
It really depends upon what you want to get access to. You are a little vague, are you interested in capturing the image stream or more interested in controlling the camera's (pan/tilt/focus etc)?
Hijacking the image stream may be straight forward, but it depends upon the web-enabled interface for your camera/controller. A lot of IP camera's will expose either a video stream or constantly write to the same jpeg file. You can easily build an app that consumes the stream and extracts the stills for image processing or just constantly read the same (but updated) image file for your processing.
You'll probably find that the camera vendor will provide an API and documentation if the camera does not use a standard format. This should be one of your first ports of call.
In the past, i've hooked up a small app to web-enabled webcam for some rudimentary motion detection (ie very slow rate) and found Aforge.net handy.
If you are a little clearer as to what you want to do, you will get a better answer (including what language/platform etc).