I helped a friend buy a new laptop that came with 4GB of memory and Vista 32-bit OS. [ed: I've twice bought refurbished laptops from Dell and have not had any problems. This one is $600 less than a new model!] Unfortunately, the OS reports that only 3.5GB is available. So what happened to that last 0.5 GB? Here’s an excellent explanation. To summarize, the computer pretends that all memory and devices (graphics card, PCI cards, BIOS, etc) are in the same flat 32-bit address space. The OS reserves the top portion of the address space (that last 0.5GB for me) to communicate with these other devices (read this); therefore, it’s not mapped to RAM. Apparently, XP once supported PAE, an Intel hack that fakes support for 64GB, but removed it with SP2 and 32-bit Vista because poorly written drivers were causing system crashes. 32-bit Linux, on the other hand, supports PAE and can use up to 64GB of memory. Why didn’t MS allow people to turn on PAE at their own risk? Probably so they could charge more for server OSes.