Are you trying to run Media Center off of an existing desktop or something?
I suppose it would work though I wonder if you would have window focus issues. I've always run dedicated boxes so I haven't tried it myself.
Guess your question is directed to me. So -- Yes.
I started fiddling with tuner cards around 2000. It probably wasn't until mid-decade that I'd managed to integrate one or more with what I then called my "stereo" rig, although I'd achieved a configuration with the cable-provider's STB and coax cabling using XP MC Edition and then Vista-64 Ultimate. By the time I'd got everything totally "HT-enabled," I was doing it with at least one computer in a tower case. The focus of my obsession and fascination had never been "Home Theater," but merely fast computers.
Maybe I said it here or in another thread: If this had been valuable experience for me, it might prove useful to others. And I fully appreciate the quintessential HTPC in its brushed aluminum cabinet. I just never thought I needed to go that route.
So to answer your question, the focus issue isn't really much of a problem. It would be an initial annoyance -- true. On a dedicated HDTV output for Media Center with all the right choices made in the software, full-screen means that you can't sit at the desktop and get focus back to your desktop monitor without either cycling "Alt-Tab" or restoring Media Center to a windowed mode, allowing you to drag the mouse to the other display.
So I discovered a couple things. You can drag a Media Center window to fill your HDTV screen, and leave it in windowed mode. This hasn't seemed to be an obstruction to perceptions of video quality at all. If you want to switch into full-screen mode, a quick double-click to the green button should do it, and you then would go back to either Alt-Tab and "restore" for the Windows taskbar item, or fiddling with the display and mouse to get back the window.
I discovered something else, too. It is certainly possible to have a multi-monitor setup that mixes monitors with different resolution settings. I had done this with a non-HDCP desktop 4:3 1680x1050 monitor and the HDTV (HDCP-compliant, 16:9 and 1080p). But suppose you want to load Media Center to a window which is precisely set to the size of the display? You would orient the MC window to fit the upper left corner of the HDTV -- no doubt -- and then drag the lower right corner of the window to screen's edge.
With the mixed-resolution multi-monitor setup, starting Media Center always produces the MC window matched to the upper left corner, but the window is always smaller than the 1920x1080 display. So you go through the tedium of dragging the lower right corner to enlarge the window to near-full-screen. With multi-purpose, multi-monitor configurations with the same resolution, this problem disappears, and if you left your last WMC session in windowed mode on the HDTV, restarting WMC will open it in windowed mode with the window fitting the HDTV screen the way you want -- and without the tedium.
Now I've yet to find out how multiple resolutions play out with all-digital DVI/HDMI monitor and HDTV connections. But at this point, I don't have a lot of desire to find out. Still, it's not a difficult experiment, and I'll get around to trying it.
Right now, I'm happier than a pig in shit because I've completely purged "incomplete HDCP compliance," and I can view WMC either at the desktop or on the HDTV. I can have WMC active on two machines, with one producing output for the HDTV and the other at the desktop.
With the "Audio-Renderer-Updater" WMC add-in (a French app), I can switch between 5.1 analog and 5.1 HDMI->AVR audio. You could say my "klooge" has evolved, with less-than-deliberate planning, but I think I'm on the verge of a very flexible arrangement in this room.
ONe more thought. With a powerful enough system -- mine are all quad cores -- WMC is a low-demand background process -- using at most 5 to 8% of CPU clocks. I can leave the WMC->HDTV running while playing a game on the desktop monitor switched to the same computer producing the HDTV feed. And the systems take everything I throw at them in that regard.