OK. Lets try to reset the Finder to see if it helps. So follow these steps:
1. Close an running applications.
2. Click on the bare desktop and select the Finder menu item 'Go' (while holding down the 'option'/all' key). In the 'Go' drop-down select 'Library/ . Then your Home Library folder will open.
3. In the 'Home's 'Library' folder select 'Preferences'.
4. In the 'Preferences' folder find the file 'com.apple.finder.plist' and move to the Trash.
5. Reboot and upon the reboot open the Finder's Preferences and reset to your liking.
The reset of the Finder's Preferences might help.
Edit: If you are wonder sometimes bad written third party applications mess with the wrong settings or your hard disk is just starting to go south. Only time will tell.