There are very reasonably priced video capture cards available that do everthing you need. Hauppage makes one, and several others, all based on a Conexant CX878 , BT848, chips or members of its family. There are quite a few software packages available for this chip, and one feature that is common is motion sensing capture. You basically define an area in the cameras view that you want to monitor, and any motion in that view will trigger the software to start recording.
These cards are used for computer based security systems that stream jpegs or MPEG2 video to the hard drive, and also allow remote monitoring via modem.
The same family of chips is used on many PC video cards for TV tuners, and is very well supported with developement kits from Rockwell/Conexant.