I Don't know 100% about tech stuff with computers and stuff but i doo know enough about computer tech stuff. If your running 32BIT Vista thats most likely why. My old pc has Vista 32BIT with 4GB's of RAM but only noticing 3.8GB's or RAM. It's strange to me. What i was told is 64BIT for Vista is like having 32BIT for Windows 7 for the RAM to notice. I Honestly don't know. But thats i can only think of. But wait until phobus to reply. He replys to tech stuff all the time. He's a tech master. But add more details about what your running?