Filtering attachments with a virus

We’re currently being hammered by email containing the “KLEZ” and “YAHA” viruses. I’m wondering if there is any way to set up some form of content filtering in Exchange 5.5 to block email attachments with certain filename extensions (IE: .bat, .pif,.exe, etc.)?

If there’s no way of doing it through Exchange, does anyone have any sggestions for “free” or reasonably priced third party software that would provide that functionality?

Any assistance would be appreciated.