As part of an on-going upgrade of my computers (XP is not being supported by Microsoft after April 2014) I upgraded my APRS computer to Windows 7. UiView would not talk to the TNC and reported that either the COM port was busy or the TNC was sending the wrong responses.
Nothing I did would persuade the computer and TNC to talk to each other. So, nothing left to do but ask Mr Google.
After much reading, it seemed that the problem lies in the age of Uiview and it was never designed to run in Windows 7. However, setting the program to run in Compatibility Mode, choosing Windows XP SP3 and ticking the "Run as Administrator" box solved the problem.
The computer is now talking to the TNC and Uiview is running happily on a Windows 7 computer.
Sigh of relief! :-)