ACA - Agent Character Animation File is the file that holds the animation information in Microsoft Agent, a fun and useful creative tool developed by the Microsoft Corporation which features character animation technology that is interactive. The program contains data that defines how various characters will move and speak. They can move around your PC desktop at random or with specific gestures which are both entertaining and fun to watch. Many people use them for their websites, to welcome visitors by programming them themselves, as I have done, using a complementary software known as MASH by the Bellcraft Corporation.
MASH is not necessary, though to run MS Agent. MS Agent is a standalone program and was built into previous versions of Windows. You can change characters at will, have them perform certain actions, snooze, zoom across the screen, talk to you and more by right-clicking on the agent character.
The other file that is associated with Microsoft Agent is known as an .acf file and this file contains the character data. This is important to know, especially if you wish to download the other various optional characters such as Peedy the Parrot, Merlin the Wizard, and others. Once you download them, they will be unzipped from a zip file and stored on your computer's hard drive as .aca files. If you ever lose one of your downloaded files, simply do a search for .aca or .acf files on your computer and they should come up and show you where they are stored. The .aca and .acf files work together to produce the characters with animated movements across your screen.
NOTE: As of this writing, Microsoft Agent has been retired with the release of Windows 7 and 8. However, you can still go to to the Bellcraft Technology site and download MASH, the complementary program which takes the program further and is a standalone program, even if you do not have the original MS Agent on your system. I spoke to the developer of MASH several times, Gordon Bell, and if you go to his site, he can tell you more about the program. He owns the licenses for all but 4 of the characters, Al, Robby the Robot, Merling the Magician, and Genie. I am a digital filmmaker and he even gave me written permission to use the program in an animated film.
ACA - Agent Character Animation File, along with the corresponding data .acf file, creates cool animated characters to live and play in the Windows environment. Hopefully they will bring something like it back in a future Windows edition.