Blue Screen Prob

My system is Samsung F3 1TB, Gigabyte P55 UD4, i7 860, GSKill 2000mhz DDR3, 750w Corsair psu.

OK i have had some problems lately with my pc not finding my optical or sometimes it can just disappear in mid use. I have also had 1 or 2 errors one of which told me to back up my data because my hard drive is about to fail (it didnt). I noticed it was going really slowly and seemed to take ages to do anything, after checking it out i noticed that windows 7 is only finding or using 256mb of my 4GB RAM. I tried to increase the amount in the msconfig BOOT/ ADVANCED section but then it had trouble booting and just reverted back to 256mb.
Now it wont start in normal mode, it just keeps blue screening whenever i try.

I can boot into safe mode with networking hence how i am doing this post, but i am at a loss as to why it keeps BSOD in normal mode.

How can i sort out my memory recognition problem ?

If i need to repair using the windows 7 disc will that fix the problems if it turns out to be an operating system is at fault ?

I have tested my hardware as best i can no probz with the RAM or the F3 HDD or the cpu, M/B could be at fault but its all working in safe mode so some feedback is in order.