How to find out if a specific file already exists, directly from within mathcad

Can soemone tell me how to find out if a file exist or not directly from mathcad. For example, I hace a mathcad file name "mymathcad". When "mymathcad" is open I need ti check if the file "c:\myfile.xls" exists or not.

Presently I am doing this with an Excel macro but I also don't know hwo to excecute an Excelmacro from within a mathcad file.

Thank for the help

Youki