Are you trying to restore a backup of an ACT! database? If so, what version of ACT! were those backup created in? If they were created in ACT! 6.0 or earlier, then you should be able unzip the backup (using Winzip or another extraction tool), then open the .DBF file for the database directly in ACT! 2005. If you are opening a .DBF file in ACT! 2005, then you will need to change the "Files of type" selection to .DBF in the Open Database dialog box.
Note: Effective 6/1/13, Sage no longers provides support for the Act! software. This is now provided by Swiftpage.