I am very fed up with the GA-P35-DS4 rev 2.0 board I got around Christmas time. I began noticing problems after installing Windows Vista freshly on the PC I built around the board. The first manifestation of the problem was that certain applications would not install, instead giving CRC errors. It is now months later, and I have played around with this issue enough to understand most aspects of it but not what it boils down to other than a problem with the board or the BIOS. Other problems that may or may not manifest at times include: any RAM testing tool will claim my RAM is bad even though I know it to be good; Windows will sometimes not install; but 100% of the time (when I have particular CPUs in the board) certain games (including Neverwinter Nights 2, Crysis, and Bioshock) will not install, returning CRC errors and claiming the installation media is bad. Now, I have noticed when I put certain CPUs in the board (I have had the benefit of testing this with four different CPUs) the problem *never* manifests. The two CPUs which I can guarantee will have problems in the board are an E8500 C0 and a E6850 G0, both of which claim to be supported by the BIOS (I'm running F12 now). The two CPUs for which it didn't manifest were 2 different models of Conroe-L Celerons. I have tried this with many different sticks of memory (currently running four 1GB sticks of Corsair Dominator 8500C5). I have Windows Vista x64 SP1, but the same issues come up in XP 32-bit SP2. My power supply would appear to be powerful enough (a Seasonic S12 650Watter). I haven't had reason to vary the graphics card (a GeForce 8800GTS 512MB). This problem seems pretty strange because it doesn't show up randomly like a memory error, instead it shows up at set times in set processes. i.e., I cannot play through one area of Crysis without a crash; I constantly get the same error when installing NWN2, etc. I would really like this resolved so that I can use the CPU which I bought for this machine instead of a borrowed one.
Your symptoms are certainly consistent with memory issues, so I wouldn't dismiss any diagnostic programs that show memory errors. Essentially all RAM sold these days that is rated faster than DDR2-800 is actually overclocked. This causes many problems with memory errors on systems, as most systems cannot automatically set the memory (DIMM) voltage to the higher-than-standard value that these modules may require.
What are the speed, voltage, and timing specs of the RAM you are using? If the voltage spec is not the 1.8V DDR2 standard, you'll need to go into the BIOS and set the DIMM voltage to the spec value, then save the changes and reboot.
Also, disable any "spread spectrum" settings in the BIOS, as well as any Gigabyte-specific automatic overclocking options.
Finally, given the errors you report while installing software, it is likely that many of your programs have been corrupted due to these memory issues. After you get your system stable so you don't get any memory errors in memtest86+ and can run Orthos for a number of hours in "blend" mode without errors or overheating of the CPU, wipe your hard disk and reinstall all the software from scratch (this is also a good opportunity to make sure you download and use the latest versions of all your MB and graphics card drivers).
BTW, the "proper" (matched with CPU) speed for your memory (assuming you are running in dual channel mode) is the FSB datarate divided by 2. For a CPU with 1333MHz datarate, like your e6850, run your memory bus at DDR2-667 speed. Since all data to/from RAM goes through the FSB on Intel systems, matching the memory bus and FSB speeds like this will give good performance and stability. This'll save you money on buying RAM, too.
I agree with mondoman on the ram part. I had similar issues when I first tried installing xp. I had to manually overvolt the ram (it is safe up to 2.1 maybe 2.2 v) from the 1.8 to at least 2.0 or 2.1. Before the overvolting, windows crashed after 14% install. Once I overvolted the ram, I was in good condition. I also tried lowering my ram speed from 800mhz to 667 or even 533. That seemed to help until I could determine the direct problem.
I hope this helps you get your system working properly!