Just upgraded my PC and have a ASUS H87 Mobo with Intel HD4600 IGPU as well as my original (and old) ATI RADEON PIC-e 4800 series GPU from about 6 years ago. Set up the system over the weekend with IGPU as my primary adaptor (because I needed the 3D input to the projector) and set up GPU as secondary to a second (extended) display.
Everything worked well last night but when I booted up today I could no longer play any mp4 or mkv files with either WMP12 or Nero Platinum14 MediaHome (both working last night) - there are other media players that I have (3D Video Player and an old version of Nero Showtime) that played the files fine.
When I disable the IGPU and use the discrete GPU all of media players work fine still (including WMP and Mediahome). This suggests to me that the issue is something to do with the Intel HD4600 drivers (rather than a Win7 codecs issue) but can't see why they were fine yesterday but not today - after the issues first appeared I checked on line and found a slightly newer driver which I downloaded but this made no difference.
I have emailed Intel but am not expecting much back from them - am sure they will tell me it is either a Win7 problem or an application problem.......
Any advice appreciated although I don't want to be told to install VLC etc - the fact is I like WMP and MediaHome and I they were working before they should work again!