This problem occured after 3-4 months of successful working. I spent a number of fruitless hours trying a number of solutions found on forums or suggested by HP support, all to no avail, but stopped short of the sledgehammer full reinstall of windows 7.
Examining event logs (using event viewer) showed that following a powercut truesuite.exe crashed on start up. My first failure to log on correctly followed this. As truesuite subsequently crashed everytime
I booted up and this was followed by frequent log on failures (Application Event ids 1508,1502, 1515, 1511) I removed truesuite (HP s rebadged version forget the name) using Revo uninstaller. So far so good no problems. I suspect that the crashing of truesuite and the subsequent error logging causes the registry or ntuser.dat to be unavailable to the log on process. I hope this helps anyone out there.