Which IM are you using Windows Live, AIM, Yahoo, etc.? You should be able to set it up from there.
just noticed your first post.
Open Windows Live, click on Tools and scroll down to Web cam settings. Check to see if your settings are to your liking. Now click on Actions>Video>Show your web cam your contacts should be showing.
If you need further help, open Windows Live click Help>Help Topics, then click Table of Contents at the bottom left. Scroll down to Communicate with Contacts.