Just sharing this as a lesson learned...
On Windows, when using DROP TABLESPACE INCLUDING CONTENT AND DATAFILES, the actual files on the harddrive don't get removed. Sometimes (not always) after a reboot, they get deleted. And sometimes (most of the time not) you can delete them manually.
Most of the time tough, they just stay there, and Oracle keeps a lock on them, preventing you from deleting them. Why is this? What can you do about it?
So, what I've been doing to delete these files is the following...
Once I've made sure that the datafile is no longer known by Oracle, and there is no I/O going on on the datafile, I find and close all open Handles.
To do this, I use Process Explorer (run as administrator) I go to 'find', 'Handle or DLL', and then search for the filename I want to delete. Most of the time there are two handles open to the specific file. Sometimes 4, and 2 close after about 20 seconds. If more handles are open, I would double check there is no I/O, and Oracle has no knowlege of this file. Now, just right click the handles and close them. You can now delete the file.
I've been using this method for years, and never had a problem... Yet, I have no clue why those handles stay open, and what any possible side effects might be.