This personally happened to me in my office. I never used to shut down my machine and most of the works are based on copy and paste in any IT companies, i used to heavily use copy paste thing in windows while in work .At the end of a day suddenly copy paste thing didn’t work for me. I copied things but no paste option was highlighted. That meant my whole work stopped. And to overcome this i searched for the windows Clipboard viewer.which i found in windows System32 folder named “clipbrd”. I just ran the clipbrd from the run option in Start menu of the Windows ( XP ). then i deleted the clipboard from Edit menu. Bingo, Everything was fine after that. My work resumed.