Unfortunately you can't get full path of a client's system on newer browsers.
The client system's directory structure is irrelevant to any web server and thus this feature has been withdrawn in newer browsers.
You may try -
FileUpload1.PostedFile.FileName;
But as I said, this will only work in older browsers like IE 5.5. I think even IE6 doesn't support revealing full path.
I was just curios why do you want a full path of a client's machine? Are you working on an interanet application?