Hi Jo, welcome to the Seven Forums.
When you single click an item (file or folder) which already is selected, Windows 7 thinks it as a rename command and lets you edit the name. This means that you are doing you double click too slowly, Windows thinks it as two single clicks: first one to choose an item, second one to rename.
You can adjust the double click speed. Open Control Panel > Mouse
, choose the Buttons
tab and change the speed. Next to the speed slider is a folder symbol for you to test the speed.