Vista - Why PC Camera can only capture 15fps in Vista?

I got a digital PC camera (Vimicro) which can capture video 30fps using
amcap.exe in XP. However, this camera can only capture 15fps using the same
application in
Vista.
The camera supports USB Video Class protocol and drivers of this device in
Vista&XP SP2 are system default drivers: USBHub.sys and USBVideo.sys.
Actually, I tested the device in two computer. When both of them installed
XP OS, the device can work normally and capture 30fps using amcap. When both
of them installed Vista OS(the same installation packet), the device can only
capture 30fps using amcap in one computer. Working with the other computer,
the device can only capture 15fps using the same application. The only
differecce of these two computers is the main board and CPU.
Could somebody help me with this question?
Thank you~