Thanks to both of you. The problem is that firstly the window says Do not turn off your computer. When I first turned it on yesterday at 7 a.m. after the Dell logo I immediately got the blue screen with the above wording. So I cannot get into System restore; can only turn it off at the PC. A computer geek told me to wait 48 hours, that is tomorrow morning. Ugh. Yes, I do have Windows 7 Professional.
60 hours of trying to revert. Finally turned off computer this evening and held finger on button for 15 seconds. Then turned back on and problem has gone. Thank goodness, don't have to spend money on a computer geek. Thanks for all the advice. Jean