OS X V10.6 Snow Leopard :: Apple-script To Check If File Exist?

I need to check if an other applescript put a file on the desktop.however the name varies depending on the weekday it was make. (in the original script I have:

keystroke (("week ") & (do shell script "date +%U") & (".webarchive"))  ) 

I think the script to check if the file is there should be something like this:tell application "Finder" try open "File:Path:To:Desktop:NameXX" on error Run an other script end tryend tell however I am not sure how to deal with the variable name. 

Info:
MBP, MM, MBP - 10.6 + Windooz XP on a hard partition