gigabyte GA -EX58-UD5, 2-3 gb ram problem

[white_dove]

Hello. I have a very weird problem.
I have 3 x 1 GB DDR3    ( http://www.newegg.com/Product/Product.aspx?Item=N82E16820145220 )Everything was fine, until today.
When i checked something in Ccleaner,i saw there only 2 GB,not 3. I checked in windows explorer - task manager -  same story, 2 GB.
Everest shows me that i have 3 GB,included the slots - DDR3_1 / DDR3_3 and DDR3_5
I tested with Memtest86,nothing strange,no errors. I removed one by one the ram modules,and i test them separated,in all the slots,i mean in the triple channel slots,which is DDR3_1 / DDR3_3 and DDR3_5. Every module was ok,every slot was ok,too.
So,in my opinion,no slot aredamaged,no ram module are damaged,right ? I tested one version of linux live cd.It shows 3 gb ram.
I have no clue why the OS shows me only 2 GB ram. Any ideas ? I did reset to default, the BIOS options,i cleaned the dust inside the computer,but ....same story : Windows shows only 2 gb.
Please,help me guys. I can't send it back the ram,because they aren't damage,so the mothergoard. Is very very weird.
Any ideas are welcome. Thanks in advance.
PS: I use Windows XP SP3. I saw there ,before my problem appeared, 3 GB RAM,and i am very sure what i saw  
And i tried the 3 ram modules,in the DDR3_2 / DDR3_4 and DDR_6 slots. Computer didn't boot.

[oggmonster]

Windows x86 limitation by the sounds of it. How big is your graphics card memory?

[white_dove]

I found out where was the problem.
In "run - msconfig - BOOT.INI - Advanced Options - MAXMEM " was checked ,with 2048.
To be honest,i don't remember doing that. After i unchecked this option and restart the OS,windows see 3 gb ram
But if i try to check this option,and modify from 2048 to 3072 ( 2 to 3 gb ) it doesen't work,i mean,there is automatically 2048,again. I don't understand why ,but anyway,i keep this option unchecked and everything is fine.
Thanks for your help.

[oggmonster]

Ok, glad you found the problem