RDP Will Not Use Saved Credentials (Windows XP SP2; RDP 5.1)

Hi,

I work in the training department for a software company that provides its customers access to windows 2008 server r2 virtual machines accessible via terminal services.  As these virtual environments are provided to our customers, we have very little control over the operating system and associated RDP version they use to access our VM's.  Here's a problem we've experiencedoccasionally:

-Customer is presented with a web link to a RDP file that we intend for them to run/open

-The linked RDP file contains all of the connection information that the customer should need to access our environment--except for the password which is verbally provided to the customer during the training session). Example of the RDP file below (this is copied from the actual file used by the customer having the problem; I've removed the real domain\username and web address):

redirectclipboard:i:1
redirectposdevices:i:0
redirectprinters:i:1
redirectcomports:i:1
redirectsmartcards:i:1
devicestoredirect:s:*
drivestoredirect:s:*
redirectdrives:i:1
session bpp:i:32
prompt for credentials on client:i:1
server port:i:3389
allow font smoothing:i:1
promptcredentialonce:i:1
authentication level:i:2
gatewayusagemethod:i:2
gatewayprofileusagemethod:i:0
gatewaycredentialssource:i:0
username:s:(redacted domain\redacted username)
disable wallpaper:i:0
full address:s:(redacted web address):55031

-When THIS PARTICULAR CUSTOMER opens the file, none of the web address or username info is present on the RDP connection screen.  If the user downloads the file and attempts to edit the file to view the connection information, nothing is there.  The only way we can get them to successfully connect is to download the file on one of our internal PCs, edit the file to view the correct connection information, and paste thatcorrect information on the general tab of the customer's remote desktop client using a screen sharing service.  Very frustrating for us and the customer.  

-This particular customer is trying to connect to our VM's via a windows xp SP2 box with RDP client 5.1.  

My next troubleshooting step, independent of the advice I may receive on this forum, is to ask the customer to upgrade to RDP 6.1 to see if that solves the problem.  Even if that fixes the problem, I'd appreciate some advice on WHY the 5.1 client won't use the credentials saved in the RDP files we're providing to customers.

Thank you for your help.

Raymond