Please help me regarding on the memory.

Hey guys, several months ago, my system is running all of its 4GB of RAM. Unfortunately recently, I notice some slowness of my system but when I checked the computer, the windows detects 4GB of RAM but 1.96 usable. The max memory in the msconfig is unchecked. I went to the bios and it shows 4096 physical memory. I am now confused. may you guys help me? I was watching while the system boots it shows 4096 MB OK but it shows only SINGLE CHANNEL. <<<< I WONDER!

My system info:

MSI H55 GD-65 (supports dual channel)
Kingston PC-10700 667Mhz 2GB x2 (4GB in two module)
windows 7 Pro 64bit

I want to fix this as soon as possible. I was suspecting it was a faulty ram because it somehow has history of BSOD.