In addition to all the other solutions I've read about here and elsewhere (like changing caching of thumbnails in the Group Policy Editor, which is not available to Windows 8 Home users, and so forth and so on), I've discovered that one doesn't necessarily
have to go all the way to restart, which many here and elsewhere have said is the only surefire way to fix this.
Instead I found that if I simply sign off my computer and then sign back in, the folder/file is also unlocked and can be deleted.
Yes, that still means you have to save all your work before you sign off. But it's still somewhat faster than a full restart.
I'd like to know if others, faced with this problem, find the sign off/sign on option less burdensome than the restart option. Thanks!