Understanding Memtest?

Hey guys, I am working on a DELL XPS 400 [Windows 7]. This unit has 4 slots with 2 ram sticks 1gb each, it was booting when the owner brought it but I researched the beep codes and found it is a ram issue so I reseated the ram sticks one at a time and it booted up but I am thinking one of the ram slots may be faulty as one time it did not boot up and gave the beeps again but the 2nd time it worked, does this mean it can have an intermittent problem?

Also, when the unit is on in windows it is...

Understanding Memtest?