The other day I received an email from a Smart Home Hacks reader looking for a way to convert an analog surveillance system into one that's accessible from the web. This approach can make a lot of sense if you already have security cameras installed, or if you want higher quality video than what a typical webcam provides.
I haven't tried it, but the general approach is to use an Analog to IP Camera adapter, such as the Hawk-I. This one is among the lower-cost options, and only converts one camera. You can also find 2 channel boxes if you need to view an additional camera.