Well, if the procedure is as Arwyn the Betrayer says, then open explorer, go to C:\Program Files\Black Isle\Baldur's Gate (if installed to default directory), and under that folder you'll see an additional folder called
Override. Unzip the xpcap file and place it in that folder. Then run the game as normal.
"Override" is where most of the smaller changes for Infinity Engine games take place e.g. "homemade" spells and items. The other important files are the dialog.tlk, and the CHITIN.KEY but you don't want to know about that right? [img]smile.gif[/img]
BTW always make copies of the files before you mess around with them. I've ar$ed up IWD twice this week 'cause of not doing that very thing.