Hold down the 'shift' key and single click at the very end of what you want highlighted. You may select more than one item by click, hold and dragging so that the dashed box covers them. Good for moving large numbers of files to different folders or disks. You may also select more than one item on the desktop by holding down the 'shift' key and single clicking the items you wish to select. Click hold and drag the 'Edit' Menu.
Most programs have an 'undo' feature there.
If you do something, such as hitting return, etc. before you correct your mistake, the damage is irreversible in most programs. Quit the program you are using. If you still get the error message, repeat the steps and raise it another 500 If it still gives you the message after you have raised the preferred memory size several times, chances are you can't do what you had wanted to. When done, set the preferred size back to what it was. If the problem occurs again there is something wrong with the program, or with your computer.