I think (not sure) it's mixed in with the shared utilities and notebook utilities. However, as I read through the knowledge base articles, it appears that Sony uses Wireless Zero Config which comes with XP. You may already have the "driver". Try: Control Panel -> Network and see if you have some kind of wireless device icon shown.
Also, if you select "Knowledge base" from the menu on the left side of the screen, and inscribe "wireless drivers", you'll get some additional clues.