I had this issue a while back and I have this recollection that there is a quick but drastic solution which is to click either the Restore Defaults or the Reset all Folders button in Folder Options. Unfortunately I cannot remember which one but maybe someone else will!
Open MyComputer (in any view)—Tools—FolderOptions—FileTypes, find the file type called Folder—click on advanced. If none of the options are in bold try selecting open and explore and then click "Set default". Then return and see if either of these settings corrects your problem.
There is also a possibility that a registry entry is corrupt and the above may not work - if so try this sololution HERE .......AT YOUR OWN RISK