Right-click on Computer and select Properties then Advanced system settings. On the System Properties dialog, go to the Advanced tab and click on the Settings button in the Performance section. On the Performance Options dialog, select the Visual Effects tab. There are four radio buttons there plus a list of selectable options. Depending on which of the four radio buttons you have checked, different options will be checked in the list. In the list, find the option "Show window contents while dragging" and make sure that is checked. If it is not checked and you check it, the radio button above will change to "Custom:".
I personally check the radio button labeled "Let Windows choose what's best for my computer".