I tried to activate DES System yesterday, and a few hours after, I began to experience random freezes on windws which forced me to reboot each time.
I thought the problem could come from the RAM (2 gskill 1gb pc6400) and a memtest run gives me a lot (35000+) of errors, but it is strange that 2 modules could fail at the same time.. I tested both separetely and there were still a lot of errors. Furthermore, I ran a memtest with this ram on another computer, and there weren't any failures.
Should I suspect something has gone wrong with the motherboard?
Thank you and please excuse my english

As I can't modify my post...
Here are my specs:
Corsair HX 520W
Gigabyte EP35-DS3R
2*1gb pc 6400 G.Skill
Intel C2D E8400
HDD Samsung Spinpoint 500gb

Hydex which ram banks are your ram in on p35 ds3r motherboard.

I suspect that your ram might need a bit more voltage to run properly. Go into the bios and up the ram voltage by .1 or .2 and see if memtest runs.

Well I tried on each of them (there are 4), no change.

No change (well, memtest may have run without error a bit longer (the windows version) and a bit fewer of errors (~5000) on the boot test.

Hydex did you try what geofelt suggested,what is the ram set to in bios.and also did you mean you tryed both 1 gig ram sticks in both of the yellow banks and then tryed them both in the red bank?

I tried both on the red, both on the yellow, then only one alternatively in each of the four banks, and then the other one in maybe one or two slots.
The voltage is +0.2, I have to verify what is the voltage now, I didn't try more.
thanks

Time for more tests!

I have 3 DDR2 sticks, 2G.Skill 1gb, 1 Noname 1gb bought cheap for testing. I took the MB out of the case and put it in its original box, I only plug the Graphic Card and the sata DVD Writer (to use memtest which is on a cd).
Yesterday I did some testing, and the results are totally strange for me :
Noname stick in bank 3 : Lots of memtest errors from the beginning, I reboot
Noname stick in bank 1 : No memtest error for 0:59:00, then freeze, I reboot
Reboot loop, nothing on the screen
Noname stick in bank 2 : same loop
Noname stick in bank 3 : Boot and no memtest error for 1:20:00, I stop
Noname stick in bank 4 : Boot and no memtest error for 1:00, I stop
Noname stick in bank 2 : Boot and no memtest error for 0:30, I stop
G.Skill 1 in bank 1 : No memtest error for 30 minutes
G.Skill 2 in bank 1 : No memtest error for 30 minutes
G.Skill 2 in bank 1 and G.Skill 1 in bank 3 : I let the memtest run this time. No error for 3:06, then FREEZE.
I don't reboot just after but a few hours later, no change in the sticks, boot and no errors for now 45minutes.
This is SO random. I don't understand anything
Any idea?

Hydex check around the motherboard and check the caps on the motherboard,they are cylinder shape and they may have bulged on the top of them,after that,if you can get another motherboard (borrow from friend) you could try that also.hope this help you.

The capacitors on this board are all solid, major53 - they shouldn't be leaking, though I suppose they could be completely shot.... then they'd likely look slightly charred.

Hydex, try to make sure you have your memory timings loose (try 5-5-5-15 at +.2V on the memory) or set to Auto. Also, make sure the memory multiplier is such that your memory is only running at 800MHz at most.

Your errors seem strange and slightly inconsistent. I hope you don't have any of the auto-performance features running - those might mess things up. Turn those off - I think Gigabyte calls it "Standard".