Hosted spam and av filter
How about a hosted av and spam filtering. We use a company called email filtering system from Email protection agency. Basically they provide a hosted av and spam filtering which you can control to create rules etc for your domain.
The way this works is that your mx records for your domain points to them and any email that comes to your exchange box is already scanned for viruses and spam filtered. Your exchange box uses the hosted solutions' MTA as a smart host to forward email and it recieved emails only from the hosted solutions provider's MTA so you only get screened emails which is virus and spam free.
The cost is not too bad either and you also don't need to worry about installing anything on the exchange box other than the exchange solution and a file system AV to protect the file system.
EPA - Email Protection Agency | Email Filtering Services
They also do archiving as well so all emails sent to your domain is archived even if they are spam etc.