Enable support for 4GB of RAM (or more) in Vista 32-bit

On a computer that has 4 GB of RAM, the System Properties dialog box and the System Information dialog box may report less memory than you 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.

The pae parameter enables Physical Address Extension (PAE). On 32-bit versions of Windows, PAE is disabled by default. PAE is an addressing strategy that uses a page-translation hierarchy to enable systems with 32-bit addressing to address more than 4 GB of physical memory. PAE also supports several advanced system and processor features, such as Data Execution Prevention (DEP; "No execute"), Non-Uniform Memory Architecture (NUMA), and hot-add memory, so it is also used on computers with less than 4 GB of memory. PAE must be supported by the processor.

On a computer that supports hardware-enabled Data Execution Prevention (DEP), PAE is automatically enabled when DEP is enabled and automatically disabled when you disable DEP. To enable PAE when DEP is disabled, you must enable PAE explicitly: Open an elevated Command Prompt.
Type BCDEdit /set nx AlwaysOff & BCDEdit /set pae ForceEnable and press Enter.

Info & Sources:
• Memory Limits for Windows Releases
• Windows Vista may report less memory than you expect
• Boot Parameters to Configure DEP and PAE
• BCDEdit /set Parameters