Windows 7: Win 7 freezes on video playback in media center

A week ago I was downgrading the video card in my HTPC from a Radeon HD 4870 to HD4850 to use the 4870 in a crossfire setup in a gaming pc. After installing the HD4850 in the HTPC it started freezing during while using Media Center- especially during video playback or live tv viewing. I have Hauppage 2250 and 950Q
tv tuner cards installed though at the moment they are removed so I can try to isolate the issue. After switching back to the original HD4870 and doing a fresh install of Windows 7 on a new hard drive the computer still freezes during video playback. I just learned about this forum, so I am hoping someone can help me out. I've attached my BSOD files below. Thanks!

Windows 7 Ultimate x64 installed OEM version on a home built HTPC.
The hardware is 3 years old. I reinstalled the OS 2 days ago on a new hard drive .

Without digging through all that debug info it would be much easier if you just described the problem. I'm guessing it's codec related without digging through that mass of debug info.

Media center will record shows without any problem but if I go to play a recorded show or watch live tv the whole computer freezes and the speakers make a a buzzing sound. The problem started after switching the graphics card so at first I thought that the new card couldn't handle the workload so I put the original card back in but the problem persists. I can also get it to lock up during playback of video files using vlc player, hulu, and abc online, though it seems to take longer to lock up (45-60 mins) when watching the online shows than media center which will lock up almost immediately.

Well what I am seeing in the list of processes is that you have ATI client items running but MOM.exe is nowhere to be seen. It is a basic part of the ATI management system and could be leading to video problems. Also when swapping graphics cards did you drop into the bios and reset the escd? ('configuration data' some bios have it listed as that) If the swap started the problem and the swap back didn't remove it. A simple cmos/bios stuck on stupid problem could have occurred. Other than seeing the ATI items and not the mom.exe I see nothing in your processes or in the error report that leads me to think it's a software issue, but rather some sort of hardware irq / assignment problem.

Dump caused by your ATI Graphic Drivers

- Boot into safe mode with networking
- Uninstall the ATI drivers including Catalyst Control Center (in Device Manager)
- Use driver sweeper to remove leftover ATI display driver files Guru3D - Driver Sweeper
- Reboot in safe mode with networking ATI Radeon (Click on the first one, "Display Driver")
- Install the latest drivers
- Reboot into normal mode