With a Windows based solution, you will have to pay a lot for OS license fees. Instead, doing this on a few Linux boxes is more efficient and cost-effective.
Install XawTV. It should come with a binary called streamer. Streamer can capture video from a video card or a web cam. It uses only a little amount of CPU and RAM per channel.
For example,
streamer -q -c /dev/video0 -f rgb24 -r 3 -t 00:30:00 -o /home/vid/outfile.avi
will record half an hour stream from the /dev/video0 device and save it to an output file specified by -o. You can write scripts (bash/perl/python etc) to do the recordings automatically (invoked every half an hour from crontab, for example).
With ffmpeg, another open source application, you can convert your recorded file (avi in the above example) to most popular compressed formats (both audio and video), including the windows video format (wmv), and mpeg.
Hardware-wise, there are capture cards that can handle 16 video streams with audio simultaneously. But I recommend 4-channel capture cards, as these will provide better image quality for TV. The others are more suitable for low quality surveillance camera recordings. There are vendors supporting Linux, with their own dedicated Linux drivers. You may have to check if the card can work with XawTV/streamer. BT787 is a pretty standard chipset that is supported by all Linux flavours. Beware that not all video cards support audio input, and in that case, you would have to use the microphone-in of your computer for audio, which in turn restricts the number of audio channels you can monitor to the number of audio cards you have.
