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You’re probably wondering why I picked a number as horrible as 3.5GB? Why not 4GB? Well, there’s a really technical explanation for this but to make a long explanation short, 32-bit operating systems can only handle up to 4GB of memory. Along with the RAM, you also have other memory-mapped devices such as your video cards. The memory used counts towards the 4GB virtual memory address capacity of a 32-bit operating system. But actually, the world does not end at 4,096 megabytes for 32-bit Windows Vista users anymore because the Arsgeek is here to save the day.
If you’ve accidentally bought 4GB of RAM, this may be a good idea. And I say may because many 32-bit operating systems begin to lose efficiency as they surpass the 4GB mark. For that reason, if you have less than 3GB RAM, it’s not necessarily the best idea to go out and buy some more RAM. If you want to utilize more RAM (between 8GB and 128GB), you can always switch to 64-bit Windows Vista. Alright, enough babbling. Let’s get started.
Steps:

1) Access cmd: Click on the Start Pearl > type cmd in the Search Bar > and press Ctrl + Shift + Enter (this allows you to run cmd in administrative mode)
2) Type BCDEdit /set PAE forceenable

“BCDEdit is a boot configuration editor for the command line. Using the above command you’ve just enabled Physical Address Extension (PAE) which can address memory larger than 4 GB. ”



Source:How to Enable More Than 3.5 GB of Memory in 32-Bit Windows Vista · Vista Rewired