I'm such a forum virgin! I don't know the details about posting, replying to a post or how to use BB code so, this is going to be a quick note to thank you for your response and the information.
To quote you: "Just close Firefox before running CC."
My problem is knowing how to close Firefox because, since Firefox 3.0 was set as the default when it was downloaded, it starts running automatically each time the computer is turned on. When I hear someone say that they "closed their browser", I assume it means that they didn't log in (or, if they were logged in, they made sure that they logged out). Before starting the CCleaner scan, I checked the Task Manager and learned that Firefox, as well as various other programs were running. However, I went ahead with the scan, which produced the error message that I mentioned in my post.
Anyway, thanks for the support...you gave me the confidence to quit worrying so much and finish the scan. I was concerned about how long of a list the scan had produced but, I attribute it to the fact that I used the latest v2.10.618 CCleaner. From reading some of the other posts, I understand that CCleaner v2.10 does a more thorough job.
To quote you again: "...anything that wasn't cleaned can be swept up next time." I'll quit worrying and take your advice.