It's taken a while to figure this out, but the DG35EC has graphics issues when the system has 8GB of memory installed. When the PC boots Windows 7 (x64) it spins indefinitely on the flashing Windows logo. As I'll mention later, it looks to be handling an interrupt storm.
BIOS versions exhibiting problem: 0.118 (latest version) and 0.117.
BIOS versions NOT exhibiting problem: 0.116.
When uninstalling a PCIe GPU, I noticed the system was very slow upon the proceeding restart when running Windows. All four cores of the Q9300 were showing load of >50% and the system was as slow as molasses in the fridge. I managed to open resource monitor and could see the interrupt and DPC processing as consuming much of this. Rebooting the machine, the system was never able to even reach the Windows logon screen.
After disabling assorted devices, I tried rolling back BIOS versions to see if this made any impact. The system was on 0.118. Uninstalling 4GB of RAM and re-install it around each attempt. The error looks to have been introduced in version 0.117, version 0.116 works okay.
I found someone else reported a similar issue earlier this year:
There is nothing in the 0.118 or 0.117 release notes that suggest anything to do with IGP graphics issues. Presumably this issue is a side effect of another change.
The IGP driver version is 22.214.171.1240. This is the latest available from WHQL.
It seems the issue could lie with either the graphics driver or the BIOS. It would be great if a proper fix could be found for this issue. Failing that could you stick a big WARNING LABEL on those BIOS versions?