Originally Posted by Agent 
...the /PAE switch is not used in XP-SP2...
Actually, PAE is enabled by default in XP SP2, this is because DEP is turned on and requires PAE.
If you check the properties of My Computer you should see "Physical Address Extension" mentioned.
I set up a virtual machine in Hyper-V, gave it 1GB of RAM and installed XP SP2 to verify this - here is the kernel used according to the debugger:
lkd> lmvm nt
start end module name
804d7000 806cd280 nt
Loaded symbol image file: ntkrnlpa.exe
The "pa" bit indicates a PAE kernel.
I added a second boot option in BOOT.INI with "/noexecute=alwaysoff" and checked again - system properties no longer reads "Physical Address Extension" and the debugger shows a different kernel:
lkd> lmvm nt
start end module name
804d7000 806eb100 nt
Loaded symbol image file: ntoskrnl.exe
ntoskrnl = uni-processor, non-PAE kernel.
Next I added a 3rd boot option with "/noexecute=alwaysoff /pae" - back to "Physical Address Extension" visible on My Computer properties, and the debugger says:
lkd> lmvm nt
start end module name
804d7000 806cd280 nt
Loaded symbol image file: ntkrnlpa.exe
If it were a multi-processor kernel with PAE enabled, the nt module is ntkrpamp.exe ("pa" + "mp").