Webcams gives nothing but black screen

Hi, I have tried (on separate occasions) two different brand new webcams on my system (Windows XP Home) and neither appear to work. They just provide a black rectangle on the screen. The webcams in question are 1) Phillips Toucam and 2) Silvercrest USB Webcam. They attach via a usb hub. They can be seen on the hardware list but that's about it. Can you advise please? Thanks Kernow

Did you try them connected directly and not through the hub?