I have recently installed a Logitec Quickcam Pro 5000.  Prior to
installing the Logitec software the sound on my system was fine, after
installation the sound is full of crackle/interference, this is for
all playback not just the webcam, WinMedia, iTunes etc.  As soon as I
uninstall the webcam everything goes back to normal and the sound is
fine again. I have a C-Media AC97 Audio Device and have ensured that I
have up to date drivers for both that and the Webcam.  Any help would
be great!

Install it again amd check the device manager for any conflicts

Hi scaramanga-ga, 

First, it's recommended not to use the onboard sound simply because
its thin and tinny sounding andcompletely inadequate for playing music
and films.

Are you using the latest motherboard drivers from the manufacturer
and not just the ones in Windows? Same goes for your soundcard.

I'll recomment to uninstall both webcam and audio drivers, reboot the
computer re-install the audio & webcam drivers.
Please pay attention to the driver version you are installing for both
devices. I've encountered the same problems with this audio driver and
had to uninstall is several times untill i've found a perfectly
working driver.

Check your audio driver version & build with the following links:
http://www.softwarepatch.com/utilities/cmedia-9738.html
http://list.driverguide.com/list/company222/index.html
http://www.opendrivers.com/driver/211125/C-Media-9738-9739-AC97-Codec-UDA.html
http://www.soundcard-drivers.com/drivers/145/145142.htm
http://www.soundcard-drivers.com/drivers/136/136560.htm
http://soundcard.free-driver-download.com/C-Media/16817/C-Media-AC97-Codec-Audio-Rack-2.11-Windows-98SE-ME-2000-XP.html

Regarding the Webcam driver:
You must have DirectX 9.x or higher and Internet Explorer 5.5 or
higher installed prior to installing this software.
Note: This release is intended  for the QuickCam Fusion, QuickCam
Orbit MP, QuickCam Pro 5000 and QuickCam for Notebooks Pro only.
Download the latest driver compatible with your webcam here:
http://www.logitech.com/index.cfm/downloads/software/US/EN,CRID=1794,contentid=11022

* {If the problem persists then mabe your problem is one of latency
and some device trying to hog resources.
Theres a utility called PCILatency
http://downloads.guru3d.com/download.php?det=951
that you can use to force devices to share the bandwidth equally.
For example graphics card sometimes get a latency of 200 or more and other
devices only get 32
- equalize the latency at 32 and all devices get an share}

Hope this will help.
Best Regards,
Bernard L.