Obtain UserID from Credentials rather than Windows Login

I am using the UserID parameter in RS2005 to run a stored procedure from my datasource and have set up the report to connect using Credentials supplied by the user running the report.

I have put the User!UserID field onto the report and when I run it from a colleague's Windows account, using my credentials, I get their name returned on the report.  I need the UserID being returned to be that entered in the credentials.

Is this normal behaviour?  Have I set something up wrong? How can I get this scenario to display my name?