I am trying to run BOINC on my HTPC running Windows 7 x64 Media Center. Here's my problem. BOINC is set to start after 5 min of inactivity. When I am watching a movie, etc. then it is assumed I am "inactive", BOINC kicks in, and my video playback gets choppy. The problem is worse when using a Media Center Extender, as the use of the extender does not count as "activity" that would suspend BOINC.

Ideally I'd like a setting where BOINC only runs when the CPU usage (from non-BOINC stuff) is below, say, 15%, and every time the usage goes above this, BOINC quits. Or does anyone have another solution?

Use <exclusive_app>, which is an option in cc_config.xml
There's also a<exclusive_gpu_app> for suspending gpu work.
Note that this file is NOT automatically created. You have to do that yourself.

Wiki file on cc_config is here.

Hmmm.. exclusive_app won't work, because the media center application is always "running" - I need BOINC to stop when the executable is putting significant load on the machine, rather than just idling in the background