Hi Nataraj,
NIDS has a list of signatures that matches viruses/worms/trojans attack. You can view full list via IDM.
NIDS also has more and updated signatures compared to PIX (small portion of total IDS signatures).
You should use NIDS to monitor viruses/worms activities, and constantly load your NIDS with new signature update. You may use IEV as monitoring tool, Cisco Threat Response-IEV or VMS software that bundled together with NIDS (VMS Basic).
Rgds,
AK