Hi guys,
What you need to do is to install IRserver suite and then configure it to do what you require here.
Once IR server suite is installed you need tray launcher (to start Media Portal from) and translator (this is the bit that can change your button config once a different app has focus.
I had it running with PowerDVD and KMplayer with there respective buttons mapped.
At present i have a few issues of my own since a new install with the latest versions. I will have to read ALL the way back through the IRserver suite posts to find out what has changed ???
Heres a link to start you off IRserversuite
Good luck guys.
PS to stop the windows Media Center from running you need to use the group policy editor to stop the program running. I know that others are saying stop this and that, and effectively stopping everything, BUT i am sure that the IRserver suite uses the buttons through windows ehome so they may be causing there own issues that way.
Like i said above i just went into MMC and the group policy editor and stopped WMC from starting.