7800gt 256mb GeForce? First, make sure your drivers are all updated (just go to the nvidia website). Also make sure you have the most updated WDM drivers installed. Those are the drivers needed to make the tv tuner thingy work for your card. For that card, go here:
http://www.nvidia.com/object/wdm_geforce_4.13.html . Make sure you update the ForceWare drivers before the WDM. What is really cool is you can use your video in as a webcam online (at least I think its nifty). As for programs used for viewing what you have plugged in - there are many. TV Tool, Video Capturix, WebcamXP(makes a server to broadcast your inputted media over the internet). Ulead Visual Studio works well for me, and it came with my card.
A problem you may encounter - a black bar accross the screen when you try to view certain things (video tapes through a VCR is an example). Supposidly there are ways around this. I have gotten around it my own way through hardware manipulation. Just a heads up.