How to set default logon domain RD Gateway and/or RD Web

I'm using Win2k8 R2 servers with WinXP clients.  When I logon to the RD Web page, and again to RD Gateway, I -have- to enter domain\username to authenticate.  Anyone know how I can set the default logon domain?  My users don't even know how to spell their username most of the time, let alone what domain they're in, and I only have one domain. 

Thanks
Mark