I'm not entirely sure what you're asking, but maybe your understanding of what Windows Media Center is needs to be clarified a bit.
WMC is just DVR software that runs on your PC (call it an HTPC if you want). Armed with a proper 1 or more TV tuners via hardware (either internal, USB-attached, or network-attached) it can both (a) record programs to hard drive per your scheduling just like a real DVR, (b) play live or previously recorded programs to your PC monitor or locally attached HDTV screen as supported by your video card and drivers, and (c) deliver live or previously recorded programs to other TV's around your house via your home network.
Depending on the copy-freely vs. copy-once nature of the content of the live/recorded programs, delivery of those live/recorded programs to other TV's around your house typically is handled by a network-attached "media center extender" at that TV location, which then has its own HDMI output to feed the HDTV at that location. WMC feeds the extender via ethernet, which in turn feeds the HDTV via HDMI.
The "media center extender" can be a true extender device (e.g. Linksys DMA2100) or it can be an xBox. The Ceton company (manufacturer of internal/USB TV cablecard-enabled tuner cards to feed cable system coax to WMC for live/recorded use) has announced their own upcoming "media center extender" device, so that's one further device which will work.
Otherwise, WMC operating on your PC/HTPC is simply one more program that runs on the computer. It is ALWAYS RUNNING IN THE BACKGROUND, in support of scheduled recordings. It also supports up to five media center extenders simultaneously, which actually connect to the PC/HTPC using "remote desktop connection" technology. So again, all of the extender/HDTV locations around your house are handled by WMC ALWAYS RUNNING IN THE BACKGROUND.
The only FOREGROUND use of the WMC interface is (a) if you want to watch something in a window on your PC's monitor or secondary attached HDTV (from your video card), or if you want to get into the DVR interface to browse the Guide, or schedule a recording, or do something else through the WMC interrface.
But all of these foreground uses of WMC have nothing at all to do with what is simultaneously occurring in the background... in terms of quietly handling up to five connected extender/HDTVs where other people in your home are watching TV delivered through your LAN from WMC running on your PC/HTPC (think "client/server"), along with doing any scheduled recordings.
You are free to simultaneously use your PC for whatever else you want to do, including playing games. The WMC software is just another program that runs on your PC at the same time, certainly in the background always and often also in the foreground... all happening simultaneously along with whatever else you might be doing yourself at that PC.
NOTE: WMC actually does much more than handle TV recording tasks, although I thought that's what you were really asking about. It can also play videos and DVDs, present pictures, etc. Lots of capability other than just TV.