We get occasional calls from Mac-based corporate creative teams, asking about strange files their Windows-based co-workers are seeing on shared file servers. The files have familiar names, but all start with a “._” prefix.

Here is the explanation of where those oddly named files come from, and why they are important to the Mac-based creative team.

Microsoft Messenger 6 for Macintosh was released in September. The new version is a Universal application built to run on both PowerPC-based and Intel-based Macs, and brings a number of new features, including spellchecking, improved corporate server compatibility, and the ability to communicate with friends on the Yahoo! chat network.

There is one security feature we’ve been hearing complaints about: Messenger 6 now begins every IM session with a stern (and longwinded) warning:

Warning text: “Never give out your password or credit card number in an instant message conversation. To help prevent infection by a computer virus or worm, never accept or open any file or link in an instant message until you verify its authenticity with the sender.”

There’s no checkbox or preference to turn off the persistent warning, but if you’re willing to pry into Messenger’s application structure there’s a hack to make it go away for good.