UPDATE:
In case it helps anyone else, I finally found out what was using port 80 for me and thus blocking Wamp. It was not easy to track down, since I had already shut down all that was suggested (see my earlier post in this topic).
Here's how to find out what's using port 80 in Windows XP:
1) Go to START-->Run and type cmd to bring up a Command prompt
2) In the Command Prompt window.
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