That's cos it's the incorrect driver for the chipset... I had the same problem wheni first got my budget 6 led webcam off of Ebay... Turned out it was a different chipset driver than was actually packaged with the webcam... Handy eh???
So, i recommend (if you really wanna find out what chipset is inside the camera) to take the 2 screws out of the back of the webcam (whilst unplugged obviously) and read any codes/text that maybe on the big chip in the middle or the numbers printed on the printed circuit board.
Turned out mine was a PAC7302, which with a small look on Google, i found the appropriate drivers which work a dream.http://www.pc210.com/downloadhtml/PAC7302_PC_Camera_driver_397.html
This method is the least time consuming in the long-run, as my friend bought the same camera at the same time (from the same supplier) and it turned out to be a different chipset (a ZS211)http://download.driverguide.com/driver/ZS211/ZSMC/d1276977.html
Seems that even the sellers are not aware of what chipset they are sending out to people, so best save yourself from banging your head against the screen looking for the right driver, by simply undoing 2 little screws...