There are many resources to scan for virus and trojans, as well as those that will help give you some protection. Its a good idea to scan with multiple resources on a regular basis since one product cant be expected to find everything. If you are told you are infected, your first step should be scanning your system with 2 or 3 resources.

A listing of some resources can be found in this thread

ParaBrat @#mIRCAide DALnet