The problem could be due to two reasons:
1) Virus infections
2) Folder settings
If it is due to folder settings, you can resolve it very easily. Go to control panel-> Folder Options. I the new opening window, you will see a label Browse Folders. Under that label, you can select the option "Open each folder in the same window".
But, if is due to virus infections, you need to perform a full system scan using Quick Heal antivirus, i have used it in the same situation.