well as a medical transcriptionist it is always an issue sending confidential files...Normally we send files directly to the clients FTP which is of course very secure. However at times, I send files thourgh Yousendit, (
http://ftf.yousendit.com/?home=true).
It is free...ftp-like...and secure
For you to feel more secure, use encrypting tools such as Winrar or Winzip(free downloads) and make sure you encrypt and put a password on it.