XP won't find one wireless network, but finds others?

Hi, my girlfriends XP laptop won't see her home wireless network using its inbuilt wireless adapter, and I have no idea why! Other laptops have no problem connecting to it, her laptop will happily find the network at my house and using a USB wireless adapter it will find her home one as well, so I have no idea what could be stopping its inbuilt one from working on just this one network?

Thanks