Protocol anaylizer on switched network?

We removed our hubs and installed gigabit switches. All the servers (3 servers) are plugged into the core router which the access level switches also uplink to.

What would be the best way to scan for viruses like symptoms and troubleshoot network issues? The switches are manageable but don?t give much detail. Where not enabling vlans or anything special.

I was looking into ethereal because it?s free. Is this a pretty decent tool? Where would I need to plug into to diag the network!?

Thanks for any links/info for this environment!