Yes, XP can address 3 GB for apps, and load the kernel into the 4th GB. So it can use 4 GB of RAM, but not all of it completely.
ANy app made with Visual Studio 6.0 or later can also address 3 GB, but only after you add the /3gb switch to your boot.ini. Without the /3gb swtich, your machine will act as only having 2 GB addressable. I've known many a person that put 4 GB in their system, yet complained it didn't work. Of course it won't work, unless you tell Windows to use it!!!
-S
PS. I forgot to mention that apps made prior to Visual Studio 6 (which came out in the late 1990's) may get completely borked by having 3 GB available. And not everyone was programming on Vis Studio 6 yet in 1999 by the way. So for these apps old old old apps, you may need to remove the /3gb switch and reboot your computer to play them. Just a warning.
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