Spyware, viruses, & security : Updating and using online virus scans.

A Little Confused! What is Your Main AV? Norton Stand Alone?

..OR you don't have other than the toolbar one?
I'll state my bias right up front: I don't trust & won't allow on my machine anything including the word "Toolbar" in it's title (from 3rd party). Yahoo toolbar is listed Adware including tracking & aimed ads!
Naturally they might block any scanner that would label their toolbar a threat causing you to un-install their money maker. There are plenty of trustworthy FREE protections available without having to submit to such intrusion.

Norton, itself, is very unhappy when you try to install any product it considers as a competitor: Specifically McAfee or Trend Micro.Referred to as "not playing nice with others".
I have seen this problem here many times.

Unfortunately you ignored the sentences in RED at top of page so I can only guess at further help as you don't say your OS,SP level, other protection programs, etc. you might have.

Have you installed Sun Micro's FREE Java Plug-in? A coffee cup ICON in your Control Panel would indicate YES. If not, you should. I'd include a link to it BUT correct version depends on OS/SP patch level.

I have seen Norton block install of T.M.Housecalls using Active-X (MS's) version, but not blocking the same when using the Sun Java version of Housecalls. Their site auto detects which you have & selects Java as 1st choice if you have Sun's (or you can manually choose). Java scan is 1/2 the time compared w/ Active-X one.

With more info provided, we can offer correct advice.