Thanks for the help Tony.

I tried what you said, but when I click Properties on the file, there is no text that says that the file came from another computer.

When I go to the security tab, there is a text that says: "you need read autorisation to see the properties of this object - Click Advanced to continue" (roughly translated from Dutch 'cause my Windows is a Dutch version ; the Enlish version of this message may very but I'm sure you get the gist)

When I click Advanced, there is one of those "Admin buttons" (you know, with a little shield icon) which says, "click here to try and do the operation with a higher clearance".

When I click the button I see another menu with "advanced security settings" (see below - in Dutch, sorry about that)



When I get here, I don't know what to do. I don't see a button to "take ownership". There is a button on the bottom, which says "overname uitschakelen" (= disable takeover). If I click that, I get two options : "convert the autorisations taken over for this object to explicit autorisations" or "remove all taken over autorisations of this object".
The first options doesn't make a difference, and the second gives me a popup box that says: "if you continue, nobody will have access to this file any more, and only it's owner may change the autorisations. - do you wish to continue?" If I try that, nothing changes as well. (except I can see that all the 4 autorisations (for Admin, system etc...) disappear. I've also found out that I cannot even delete these files, since (it says) I'm not an Administrator (but I am!).

On the other hand, simply copying these files to another folder on the same harddisk (so it's not necessary to copy to a USB stick) also fixes the issue... I can play the copied file fine. But I cannot copy them back and overwrite the original file because I'm not allowed access to the original file...and cannot delete it... strange!

Edit: I found a utility called TakeOwnershipPro. Using it on a locked file does not make it playable, but from that moment on I can delete it. So worst case scenario, I guess I'll simply copy all files to a different folder, use TakeOwnershipPro on the old folder, next delete it and then copy everything back. It's a pain and will probably take me some time, but most of the process is automated so I guess it could be worse.