Windows 7 - Installed 4Gb Ram, only 2 Gb usable

Hi, new to the forums and I've been using my gaming rig for a while now. My motherboard is the ASUS Maximus III Formula, using Intel Core I5 760. And I recently did a dual boot on my system. One copy of Windows 7 Ultimate x64 on my original hard drive, and a Windows 7 Home premium x64 bit on another hard drive ( Hitachi ) and after a month or two, I had to format that ( Hitachi ) hard drive as I needed it for extra storage and now I'm getting only 2 Gb usable RAM but I installed 4Gb.

Please help

additional information

Strange, I would try clearing the CMOS, let us know if that works or not.

Do you have the same situation with the other OS? If not, it is an OS problem we have to look for.

I cleared a few times, nothing worked

I only have windows vista Ultimate 64 bit, should I install on another hard drive?

No, I don't think that would help. You could try a live Linux disk and see whether that is the same.

Also, check in msconfig > Boot tab > Advanced options. Click the little box "Maximum Memory" - what does it say there and what happens when you set it to 4GBs

When I checked the maximum memory box, it writes 4096

So there does not seem to be any physical problem - like bad connection. Do you have any idea what piece of hardware could be using the 2GBs all of a sudden.

Hmm, I might just found the cause of all this, I have a faulty DIMM slot but I don't know if I should RMA my motherboard because Im using a LGA1156 mobo. But thanks for the given suggestions and help