How can I get the sound working on my PC (Vista) again?

After installing the software which came with a logitech webcam I bought, the sound on my computer has stopped working.

When I go into the sound manager and run the speaker test, it all sounds good, but any other sound (youtube video, game, etc.) doesn’t come through.

I’m not very adept with computers and haven’t a clue where to start. I’ve done a quick google search but couldn’t find anything.

Any ideas?