On a computer that has 4 GB of RAM, the System Properties dialog box and the System Information dialog box report less memory than you would expect. This problem occurs because the address space is limited to 4 GB in a 32-bit hardware environment. Memory may be relocated to make room for addresses that the basic input/output system (BIOS) reserves for hardware. However, because of this limitation, Windows Vista cannot access memory that is relocated above the 4 GB boundary.
Solution: Open an elevated Command Prompt, type BCDEdit /set pae ForceEnable and press Enter.