use Ctrl_Alt_Del to see whether there is some bachground application is running, if yes, end it. another possibility is your system protection software is preventing you from doing scandisk. for example, in Gateway Desktops, the GoBack system software must be disabled before doing scandisk.
just remind you that even Norton Antivirus can prevent you doing the scandisk if you don't disable it. Press Ctrl-Alt-Del to see all the tasks and processes running in your PC's memory, end the suspicious one. and you had better close the explorer, although i don't think it will cause this problem.