Page "Physical Address Extension" Paragraph 4

Microsoft Windows implements PAE if booted with the appropriate option, but current 32-bit desktop editions enforce the physical address space within 4 GB even in PAE mode.

According to Geoff Chappell, Microsoft limits 32-bit versions of Windows to 4 GB as a matter of its licensing policy, and Microsoft Technical Fellow Mark Russinovich says that some drivers were found to be unstable when encountering physical addresses above 4 GB.

Unofficial kernel patches for Windows Vista and Windows 7 32-bit are available