So, your 32bit Windows 2000/XP/2003/Vista/Longhorn install is only showing 3 - 3.5GB of RAM available to Windows, when you've installed 4GB of RAM?

1. Start here, here and here for a technical overview of how this can happen, and

2. Read this, this, this, and this to see real-life examples of this happening, and what can (and cannot) be done to resolve it.


Now it's been stickied, so if you ask this question going forward, you will simply be pointed to the contents of this thread. It is NOT a Windows problem, it's a limitation of the 32bit architecture when the BIOS reserves memory for hot-add/hot-swap memory, PCI-X/PCI-E buses, and (if it applies) your 256MB or 512MB video card reserve memory resources for their usage. That means that you either run a server-class OS with /PAE enabled, or you run a x64 OS - note that even if PAE or x64 gets your memory "back" for Windows to use, you still will be missing a small chunk of memory between 3 - 4GB, because the BIOS has reserved it and the OS absolutely cannot utilize it without hanging the machine.