While I found no on/off feature for the card at http://www.newegg.com/Product/Product.aspx?Item=N82E16833130003 I continue to encounter bad XP installs. Missing drivers or folk that had no install plan.
Did you "install XP" or did you use the HP/Compaq restore media and driver CD?
My dad's computer (Compaq Evo D500, running Windows XP Home Edition) was having some operating system problems on which we had exhausted all other options, so we backed up his person files, and formatted and reinstalled windows. We got almost everything working fine pretty quickly. The computer is almost always connected by wire, which works perfectly fine right now, but when we have company, we don't like having a long wire laying on the floor, so we like having the wireless available.