If you haven’t remove the backup RTM files that saved by SP1 setup installer, there is a backup copy of USBaudio.sys version 6.0.6000.16386 in \Windows\System32\DriverStore\FileRepository\wdma_usb.inf_9ce240de folder. However, in normal running computer, the file is always in use, and thus locked and cannot be deleted nor moved while system is running. Use the trick below to copy and swap back to old version of the file.
Follow this links and check what need's to be done:
http://www.mydigitallife.info/2007/06/05/trick-to-use-logitech-quickcam-messenger-in-windows-vista/
http://www.ehow.com/how_5592388_install-logitech-webcam-disk.html
Also try this:
http://www.softwarepatch.com/utilities/logitech-webcam-driver-quickcam.html
Sometimes the issue can be because of thecorrupt driver getting re-installed. Go to device manager and unintsall if it USB webcam. and restart and check.