You can assign simply as its parents folder.
No need to mentions the drive letter in all the cases.
Look , Consider this is your folder structure.
MyProject/Flash/Index.swf
MyProject/Flash/Files/(The files which in turn open by Flash)
In this case,
To open any files in the "Files" folder from Index.swf file,
You can use the
getURL("Files/YourFile.pdf");
If you want to open any files in the "Flash" folder means,
you can
directly give the file name as argument for getURL.