Sometimes, you can restart and unlock that cache db...unless you have images on the desktop that may be holding it open. If so, you can clear it via safe mode or from a command prompt.
You can clear that directory from the command line...open a command prompt, drill down to the folder using CD. Then, open Task Manager (Ctrl-Shift-Esc), go to Details, and End Task on Explorer.exe. The desktop will "disappear" and unlock the used db files. Then, use a DEL command in the folder to delete the in-use db's. Once cleared, back in the Task Manager window, you can go File->Run New Task and enter explorer.exe to restart the shell.
It makes me wonder if that "locked" db is your issue. Either something is hooking it and not letting it go, or it's corrupted and can't update. Either could cause your thumbnails to not load (especially if that's the cache that's holding the icon size you're set for).
As far as the codec pack, it's now built into windows itself and can't be removed. Sometimes, you can do a "SFC /scannow" from a Command Prompt (Admin) (right-click on the start button to get it). If one of the codec decoder dll's has been corrupted, that can replace it with the proper version.