NOOB - populate two fields based upon the selection a user makes in a drop-down

Hi all
 
Please forgive me asking a possibly already answered question. I looked but couldn't see if this had already been answered.
 
I have a form that is tied to an xml file of data. On the form is a drop-down where the user can select a UserID. When the UserID is selected or changed, I need the other two fields on the form (UserName and UserDept) to update with the information in the xml file relevant to that UserID.
 
xml file example data:

1234
Smith, Bill
Shipping

1235
Turner, Ted
Media

1236
Jollie, A
Acting

1237
Harris, Richard
Shipping

 
The form has a drop-down for the UserID and fields for the other two values.
 
Could someone please direct me to an example of how this can be coded? I've looked at a couple of examples but they seem to relate more to arrays in the form. This form needs to use an external xml file (and will eventually use a database connection).
 
Thank you in advance
RD (grouper)