"We have found that applying Service Pack 2 (SP2) to Windows XP
imposes a ~3gb limit for memory. All 4gb will be there, but XP SP2
only "sees" ~3gb. This is called a "security fix" to ward off memory
exploits in Windows.

Running with no service pack or SP1 will allow all 4gb to be reconized
by Windows XP." -AndyT OCZ Memory
I referred him to that...we'll see if he believes me now.