Anti-Virus filter for mailenable

Hi,

I'm having some serious trouble with viruses. All our clients email accounts are automatically sending 1000's of infections emails to unknown users! i scanned for viruses but the server machine returned cleaned. I cleaned one of virus off called net_ky32? i want to know how do i stop this?? i'm using mail enable professional on windows 2000 server. I tried installing f-prot antivirus filter on mail enable pro... no luck! still having clients email boxes filled with 100's of viruses. Where are they coming from? could somebody please be kind enough to reply and provide me some suggesstions.

Thanks!