Email Virus Scanner

We use Exchange 5.5 and Outlook 2000 for email. We have Outlook configured to use the Exchange Server Service for email. I know there are programs that are loaded on the server such as Trend Micro’s ScanMail for Exchange that can scan email as they come in but it is kind of expensive for the few users we have. All of the client side virus scanners that have checked out can only scan POP mail and not the Exchange service mail. Is there a client side email scanner that can scan the Exchange email? We use Windows 98/NT/2000/XP.