Windows does not like being shut down with the power button. Doing so can cause problems. Did you install any new software or change settings before this started happening? Good chance this is caused by malware. Take a look at this
and work through the scans - you might find something on your machine you didn't invite. Checking for problems might cure it. You can also run the System File Checker from a command window. Go to Start -> Run and type:cmd
. . . and hit enter. This will open a black, DOS-like box. At the prompt, type:sfc /scannow
. . . and make sure you have the space between the c and the /.
Let Windows check for missing or corrupted files.