When I watch HD for an extended period of time (e.g., 15 minutes or more), I lose all access to the Media Center user interface--although the video continues, the remote control won't do anything except volume control (and no volume pop-up appears). The mouse works, and I can alt-tab out of Media Center.

Processor power is about 30%, memory about 60%, hard drive has over 100 GB free. Not bad.

Any ideas...? Please? :)

I'm updating my own post for the benefit of someone who happens to read this at a later time.

I found the Media Center log in Computer Management's Event Viewer, and discovered it was making a log entry along the lines of "TV tuner encountered an error". As this was the third tuner I've had in this machine since installing Vista, I thought it might have something to do with that. I browsed the registry and found several tuner entries listed under the Media Center tree, even though Media Center itself didn't list them.

I reinstalled Windows, and things seem to be working better now. I assume the problem had to do with the other tuners not uninstalling properly.