Memory Question in XP

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Hi,

Just a little question

Last week I had 512 megs of DDR 333 in my pc, the free ram was about 250 megs.

This week, I put another stick of 512 megs DDR 333 in my pc and the free amount of ram with the same process running is about 520 megs

Where is the 512 megs I have upgrade ? Is this XP taking all of this new ram ?

Thanks for your help