Some people may never bother thinking about intrusion detection if they have a firewall. However, with the amount of trojans and stuff like that going around these days, it might be worth having one.
Here are 2 free ones you can find at my pals majorgeeks.com
http://www.majorgeeks.com/Securepoint_Intrusion_Detection_System_d2759.html
http://www.majorgeeks.com/GFI_LANguard_System_Integrity_Monitor_d3619.html
This thing is lovely too - very powerful - (Also by GFI) - I use it (albeit to look at other people's computers):
http://www.gfi.com/lannetscan/?adclickid=4784848
Here are 2 free ones you can find at my pals majorgeeks.com
http://www.majorgeeks.com/Securepoint_Intrusion_Detection_System_d2759.html
http://www.majorgeeks.com/GFI_LANguard_System_Integrity_Monitor_d3619.html
This thing is lovely too - very powerful - (Also by GFI) - I use it (albeit to look at other people's computers):
http://www.gfi.com/lannetscan/?adclickid=4784848