Found my answer--thanks to all who helped out. I found the key in another thread--user who said he solved the problem by reinstalling his nVidia video
drivers. I did that, and it solved the problem. Makes sense, when you consider that the video
drivers control the HDMI port.
Another discovery: I had replaced a Vista 32-bit installation with a Windows 7 install, and I just assumed that 32-bit Win 7 would be installed. Nope--I've got an Athlon 64 in the box, so 64-bit Win 7 was installed. So, I downloaded and installed 64-bit Vista chipset
drivers (including the display
driver) from Shuttle's website. Fixed the problem immediately upon installation, with no tweaking or settings required.