Occurrences are fairly regular:
4/20/2009 9:33:50 AM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.ico" file.
4/20/2009 9:33:54 AM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.gif" file.
4/20/2009 8:18:42 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.ico" file.
4/20/2009 8:18:43 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.ico" file.
4/20/2009 8:18:47 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.gif" file.
4/20/2009 8:18:59 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.gif" file.
4/20/2009 9:02:04 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.ico" file.
4/20/2009 9:02:08 PM SYSTEM 1460 Sign of "JS:ScriptSH-inf [trj]" has been found in "httpx://www.ijoomla.com/ijoomla-magazine/ijoomla-magazine/overviewfavicon.gif" file.
What's in the IE8 browser session is Yahoo Mail, a Microsoft site, the forum page and a Sharepoint page. Understand that the traffic may be originating elsewhere.
Yes, nothing obvious there, cookies are really an issue.
So I'm not sure what else to suggest, other than monitor it as previously mentioned and if it happens again try to gather as much information as possible.