After I installed Ubuntu by using Windows installer for Ubuntu Desktop, I found that I can only run executable programs in the "Desktop" folder. In any other folders, when I typed in something like ./a.out, there appears a Permission Denied error. Also, in the "Properties" dialog box of the file I want to execute, the box of "allow executing file as a program" cannot be chosen if the file is not in the Desktop folder. What should I type in the terminal to fix the problem?