macro for checking if file exists

Hi Folks
I have an Excel Macro that looks for 4 files and modifies them like this :

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Problem is that if F1 is NOT present , it ends with error.I'd like to do something so that , whether F1 is found or not it will STILL check for F2 ..F4 . Same stuff for F2..F4
Finally I would like to Rename the file with this naming convention
Rename F1xls to YYYYMMDD_F1.xls
then invoke a zip program ( Its already there in my directory called zip.exe and i know the command line arguments to it ) to zip each file.
Thank You ,so so ,much