Here's the story:
My motherboard supports up to 3GB DDR400 ram so I bought 3 G-Skill sticks of just that
I bought the first one a few weeks ago, I had 3 256mb sticks at the time so I popped one out and put the 1 gig in it's place, leaving me with 1.5gb, it worked fine.
Well....My HD was overheating so I did a little case mod, DIDN'T touch the RAM at all, but when I restarted my Motherboard just continually beeped, after taking out my other two 256 sticks it worked fine, I figured it was mixed ram issues, no biggie.
Well.............I bought two more 1 gig sticks recently, identical to the one that's installed, but when I put them in my Motherboard once again just beeped continually. So, I took one out, same thing, so I took the other one out, leaving me with my original 1gb stick, SAME THING! I reseated it in a different slot and it worked, so I put it another stick again, the beeping happened, so I moved it to another slot, it worked, but if I put in the 3rd one it would always beep no matter what. Well, I took it out and moved the other two around and it worked fine, got a windows error message at startup "Windows could not continue to load because the following file is missing or corrupt: NTOSKRNL.EXE", replaced it in the recovery console, windows loaded ish, it got to the windows logo and froze. After a manual restart I selected "Last Known Configurations" and it started up with 2gb ram...well, the story doesn't end there...
This morning I found my PC frozen on the windows logo and restarting didn't solve the problem, even "Last Known Configurations" didn't work this time, so I removed one of the 1gb sticks and it works fine.
I want (very badly) to use the other two sticks but it seems I can't even though I should be able to, can somebody help me troubleshoot this?
The ram sticks are not bad btw, nor are my ram slots, leaving this quite the puzzling problem (or it could be something so obvious that I will never think of it....)

It seems that RAM upgrades are always easy for me except when it's my own PC.......and not just this one but various ones in the past as well....
HELP PLEASE!

Edit: here's my motherboard page: GIGABYTE - Product - Motherboard - Specification - GA-K8NS Pro

i suggest downloding memtest86 Memtest86.com - Memory Diagnostic and running a test on the ram to see it it is bad or not