I'd download the Kiwi Syslog Generator and use that to send some test messages from the 2003 servers. That would rule out the connectivity and point the issue to the LF agent. I can't find the link on the Solarwinds site but it's still on the Kiwi site( http://www.kiwisyslog.com/downloads.aspx). You could also get a packet capture to see if anything is being sent from the 2003 server to the syslog server.
If it is the LF agent is it running as the System account? Are there any "unusual" security settings on the event logs?