MSO Vista Remote Desktop does not work

I bought a laptop a few months ago to do work remotely (from home). MSO Vista's remote desktop worked fine until a week ago and then no longer finds my work computer. Window pops up saying remote desktop cannot verify the identity of the computer because either the remote computer is running a version of windows that is earlier than WIN Vista, or the remote computer is configured to support only the IDP layer. Nothing changed on either end (remote computer runs WIN XP), so I'm stuck. What do I do, please? Thanks!