moving ram around is not the same as testing it. It could be the board, it could be a power supply issue, it could be a bad DVD rom, it could be a great number of things.
Most of the time however, with issues like you are describing, it is a RAM issue. Run memtestx86+ for ~1-2 hours per GB of ram installed, with only 1 stick physically installed at a time, and then come back with us with the results. If a ram stick fails, then try the test on the same stick in a different slot, and then in a different computer (if you have one available). The idea of troubbleshooting begins with valadating what hardware works, and if you simply move things around without any actual work being done then you may miss something that could save you from needing to purchase a whole new system. http://www.memtest.org/#downiso
if you believe the DVD rom to be the issue then you are in luck! they only cost $18 for a burner these days, so simply replace it. Even if you are wrong, and there is nothing wrong with your current drive I doubt you will lose much sleep over $20.
Also, if you believe the DVD drive to be the issue you can always try loading from a USB flash drive. MS has a free utility you can run which will take a USB flash drive, make it bootable, and then copy all of the install files to it: http://www.microsoftstore.com/store/msstore/html/pbPage...
It could be the CPU overheating, or a faulty power supply.
It could very well be the CPU overheating. If there is too much thermal paste then the CPU will get really really hot, while the cooler stays just moderately warm. If there is something physically wrong with the CPU (or voltage regulators on the mobo, or voltage coming from the PSU) then it can get hot as well, and the CPU cooler will be hot to the touch. 26*c sounds like a healthy believable number though.
If the Ram turns out to not be the issue, then the PSU would be the next likely culperate, followed by the mobo. Sadly, the only way to properly test a PSU is when it is under load... and that generally requires software to be installed and running while watching voltages on a utility. Any voltages outside ~5-10% means it is time for a new power supply, it does not need to be off by much to cause instability, though it does need to be off by quite a bit to cause damage these days.
So i tried with other RAM stick (working one) and the same result . It has 4x slot and still it fail.
As about the dvd it works cuz i use it in my current system.
I tried booting from USB and the same error.
Im starting to think the board is the problem
Do you have a different board to try your current ram in to test it? If the mobo's ram voltage control has gone to pot then it can take out your Ram sticks (I had this problem with my first ex3 gen3 mobo... quite frustrating as the system worked fine for 2 months before the issue cropped up).
I would still suspect the power supply over the motherboard. Or rather, if your motherboard is having issues I would suspect that your power supply is also bad, and that replacing the mobo with a new one would just be a temp fix until the PSU blows that mobo out as well.
which HP 7800 is it? I work with them all the time, and have had pretty good luck with most of them. Perhaps check the motherboard for bad capacitors? Any pregnant or bleeding caps means that it is time to upgrade.
try resetting bios (remove battery for a few minutes), then get into BIOS and load the optimized defaults. Be sure to reload any settings that are required for your setup manually. It may very well do nothing, but worth a try.
when going through the setup process are you cleaning the drive each time? (delete all partitions, make new partition, format). I have had a few frustrating experiences where the win setup does not remove all the old files on the drive, and then gets stuck trying to boot from an install for another machine (resulting in a BSOD)... but to be honest it has only happened once.. and I think it was back in win2K days