AVerMedia AVerTV Hybrid Ultra USB TV Tuner Goes Both Ways
The $129.99 (MSRP) package is designed to work with Windows XP Media Center Edition and Windows Vista Home Premium and Ultimate Edition operating systems and includes a Windows Media Center Remote Control and receiver, so you can instantly add HDTV-pausing and -recording abilities to your MCE PC and control it from your couch.
There are less expensive (and smaller) options for getting both SD and HDTV signals to your Windows system, but the AVerTV Hybrid Ultra is a more complete solution. (Though Hauppauge has an answer to that problem as well.)