Try creating a "symbolic link" between the two names like this:
Now the same file exists with both names.
$ mv htaccess.txt .htaccess
$ ln -s .htaccess htaccess.txt
This shouldn't be a problem in general, though. Are you using any form of Windows web services like ASP pages, .NET, or the like? *nix systems do not use file extensions to identify filetypes like Windows does, so typically only a Windows-based application of some sort would create a file named "something.txt". It's also possible the developer of the application expected it to be running on a Windows server and wrote the application to use Windows file naming constructs.
If you have the application's source code, try using "grep htaccess" against the source code files to see if you can find the reference to htaccess.txt. Try changing it to use .htaccess instead.