This is a step by step guide with audio and video to explain how to use memtest to see if your memory is defect:
 
Corsair
 
 

Another thing to add is that some errors will not be found on the first pass, best to do at least 3 passes.

Thanks will fallow this steps my ram works fine but is good to know.

thanks, bookmarked the link

Windows 8 includes a simple"Windows Memory Diagnostic"  service  that will run the command during the next boot much as a chkdsk.
 

Thank you for this. I could never figure out how memtest worked.

I know this is an old thread but it, my evga p55 sli board and 4x2gb dominator ram are not coexisting right now. windows properties shows 8.00gb installed but only 3.99gb useable. 
I have checked msconfig>boot>advanced setting and my max memory check box is unchecked, so pretty sure that's not it. 
I ran the windows memory test and everything checks out but only acknowledges the 4.00gb, afterwards windows properties only lists 4.00gb.
But if I power down completely remove everything for 24 hours and 'rebuild' the computer, windows properties shows 8.00gb installed but only 3.99gb useable again.
Also it doesn't make any difference which channel any of the ram is in. only acknowledges the ram in the #2 & #4 channels as useable.

Are you using a 64bit version of Windows? If not, that's your problem. If you have a 32bit Win, it's detecting what you have installed but shows what it can use. Even then, only 3,2G will actually be used. Software limit.

What Alexis said, make sure you're using a 64bit version of Windows. If you are then I would suggest you remove the CPU and very very carefully inspect your CPU socket to make sure none of the pins are damaged as a bent pin can disable memory channels. It strongly sounds like the CPU socket has a damaged pin though, unfortunately.