In order to monitor your network and check unusual activities, you have to sniff the traffic and see for anything irrregular. IDS (Intrution Detection System) is what you need, you can span your VLAN traffic to the IDS box and IDS box has a signature database which will alert you if there is unusual/irregular traffic pattern eg; ping floods, port scan, etc.
Check the Cisco's website and you will find lots of information on Cisco IDS.
Some URLs
http://www.cisco.com/warp/public/cc/pd/sqsw/sqidsz/index.shtml
http://www.cisco.com/univercd/cc/td/doc/pcat/nerg.htm
http://www.cisco.com/warp/public/cc/pd/sqsw/sqidsz/prodlit/idsms_ds.htm
HTH
R/Yusuf