.NIT has a limit to the amount of memory it can access of the total. Theres a percentage, and then 2 GB in xp was the hard ceiling.
You could have 4 GB in it, and it would kill the app when it hit 2GB.
Also in 64 bit mode, there is a percentage of memory you can use out of the system, so I'm not sure if you can ask for the whole thing or if this is specifically guarded against.