Problem solved I hope. For some reason my Silicon Labs CP210x USB to UART Bridge driver has rolled back to version 5.3 whereas the current version is 6.5.3. This driver is necessary for communication between my USB Davis VP2 weather data logger and my PC. Seems I am not the only one to have locking up problems with these earlier drivers. No idea why it has rolled back on its own - I wasn't on the most up to date driver but 5.3 was superceded ages ago.
Many thanks again for all the suggestions.