On a few occasions just recently my win 7 pro system has come out of "screen saver" with nothing but my desktop background displayed. If I then ctrl/alt/delete the screen goes black with just the cursor showing and otherwise unuseable. After a while the background will reappear but the only way that I can get the icons and everything back is to reboot. I have done a very thorough search for malware but none found. The problem never occurs whilst I am actually using the system and it does not always happen when it has been left to go into screen saver. Any one any ideas?