turn down the gfx to there very lowest...does it still lag?
My bet is the Celeron is a bit too slow to play everything at high detail, and you probably have a number of unnecessary background services eating away at it's power too.
The video card should be able to play the game at ~ medium settings at the least i believe, that game doesn't seem too demanding. I think with a bit of XP/BIOS tweaking you can get it to stop lagging.
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Compaq Presario V5000 laptop