If this tutorial no help try reset thubnail manually -it strange but probably Disk Cleanup only empty cache files don't remove these files completetly thus clearing manualy cache files are more credible.
Run system in safe mode,navigate to C:\Users\<your account>\AppData\Local\Microsoft\Windows\Explorer
and there you'll find 6 *.db files ->remove them + restart.System re-create these files after restart
My Computer
At a glance
Windows 7 Professional x64 PLAMD Athlon 4450eGoodRam DDR2 2x2048MB (FSB 800MHz)nVidia GeForce 9600GT (G94-300) 1024 MB