Help With RAM Install SOLVED

I upgraded my RAM from 6 To 12 GB. I had  no problems installing and it showed at System Info that I had 12 GB of RAM installed. Two weeks later I added a Hard Drive for storing project files etc. No problems there. This morning my PC seemed to be running erratically so I called Microsoft Online Support. They checked out the system and did this and that but when the tech went to System Info it showed 11 GB of RAM. I told him about installing the 12 GB RAM so he checked files etc.and  said I needed to speak to the people that I bought the RAM from. He thought one of the sticks may be bad. I contacted Memory Stock support and they had me reinstalled all the sticks. After doing all that it still showed 11GB of RAM. Memory Stock support said that I have a software or hardware issue and I need to take it to a PC repair store. They said if it was a bad stick it would show 10 GB of RAM. So any help or guidance is greatly appreciated. My PC seems to be running fine but it still shows 11 GB RAM. Thanks 

First thing, is the BIOS showing the full 12GB?  Where do you see 11GB in Windows? 
 
Second, this is a Dell.  Dell's have a history of compatibility problems with some brands of memory.  There is no pattern on what model, one of a same model will work OK and another not.  What did you use for add on memory? 

memory only

He probably didn't lie.  In most cases what he sold will work without problems.   However, you didn't need the 1266Mhz memory according to the Dell specs which state "1066-MHz DDR3 DIMM; non-ECC memory only"   The kicker is the fact its a Dell and it can be finicky about memory.
 
Having the proper voltage memory is a concern.  It looks like you are OK on that since the BIOS recognizes all 12GB.
 
On my system, Windows states 16GB (what I have) with 15.9GB useable. (Accessed by Start, RIGHT click on "Computer" and then LEFT click Properties).
 
I used to have a Dell Dimension E510.  The Intel chipset that was used, according to Intel could accept up to a D960 CPU.  However, the Dell custom implementation of that Intel chipset limited it to a D940 CPU.  If a D960 was installed it would not boot.  I've seen similar issues between what the Intel specs are and a particular Dell model.