On a Mac... for a user... You can calculate it based on the get(documentsPath) ... just substitute the word documents with downloads. I can't remember specificially for windows... but here are the examples in the Help for Get(DocumentsPath) which might help:
For FileMaker Pro, for a user named John Smith, returns:
/C:/Documents and Settings/JohnSmith/My Documents in Windows XP
/C:/Users/John Smith/Documents/ in Windows Vista.
/MacintoshHD/Users/John Smith/Documents in the Mac OS
For FileMaker Server, returns:
/C:/Program Files/FileMaker/FileMaker Server/Data/Documents in Windows Vista.
/MacintoshHD/Library/FileMaker Server/Data/Documents in the Mac OS