Both answers given are correct, but I do mine a little different. You might want to consider a couple things...
Start the batch with:
and end it with
This will keep all your 'SETs" to be only valid during the current session, and will not leave vars left around named like "FileName1" or any other variables you set during the run, that could interfere with the next run of the batch file. So, you can do something like:
IF "%1"=="" SET FileName1=c:\file1.txt
The other trick is if you only provide 1, or 2 parameters, use the SHIFT command to move them, so the one you are looking for is ALWAYS at %1...
For example, process the first parameter, shift them, and then do it again. This way, you are not hard-coding %1, %2, %3, etc...
The Windows batch processor is much more powerful than people give it credit for.. I've done some crazy stuff with it, including calculating yesterday's date, even across month and year boundaries including Leap Year, and localization, etc.
If you really want to get creative, you can call functions in the batch processor... But that's really for a different discussion... :)
Oh, and don't name your batch files .bat either.. They are .cmd's now.. heh..
Hope this helps.