On Windows XP Professional installations I had a strange behavior with the mstsc rdp client.
After a while, the mstsc client expired and if I tried to start it the following message showed up:
I was pretty sure I didn't installed a pre-release service pack on that machine. The problem wasn't to fix by downloading the service pack and repetitive installation. After this I found an easy way to fix this issue by installing the new mstsc client version 7.0from the Microsoft website.