I mentioned previously that I had most of Windows Media Center 2005 going... I spent a few hours this evening and am much further along.
After some troubleshooting, I found the reason I wasn't getting video was an incompatible DVD decoder. Since I built my system using an NVIDIA GeForce FX5200, I installed the NVIDIA DVD Decoder and that turned out to be the key. Now I can playback DVD's and watch TV.
Windows Media Center
The Media Center guide is really nice - it shows a small window of live TV while you browse what shows are available.
I set it up to record all episodes of Scrubs, a comedy I used to watch and really enjoy. I only stopped because Scrubs shifted around days and wound up on Tuesday playing against 24 which meant I had to choose between the two shows, and Scrubs lost. Now, between my ReplayTV and Media Center system, I'll be able to catch the occasional conflict.
Of course, there is still a bit more to do. The current sound setup is bad - I have the computer sound output sent to two small cheap speakers. If there is such a thing as a "1/8 inch stereo miniplug to standard stereo cable" (i.e. red/white right/left cable) then I can send the computer sound out to my TV, which is what everything else is doing except the DVD player. Or, with the cable I can try piping sound through my DVD/receiver. Or, I can try the optical S/PDIF output... all this stuff will wait until I'm back from vacation.
The other improvement I could make is video output. Right now, I'm using the S-video output from my graphics card, and the video quality is acceptable. I could use the DVI output since my TV has DVI input, but I want to save that video input for my next HTPC project, a linux MythTV HDTV decoder. So I'll just leave the video output alone as I'd rather feed an HD signal to the TV via DVI.
For now, it is working enough for me to test out other features, such as recording TV!
I've tried out the TV recording function, and it works like a champ. I watched Scrubs, skipped commercials, rewound and paused, and it all worked great. The Media Center UI is very polished and easy to use, definitely on par with TiVo. It has been a year and a half since I've had a TiVo (due to bad luck with hardware both TiVo's I had died) and I don't remember navigating the UI as well. From what I do remember of TiVo, both it and Media Center are a notch above ReplayTV. I might have to split hairs to decide between TiVo and Media Center though.