Try this - it worked for me when I had a file open using MS Word.
1. Open a window showing a directory other than the directory the file is in.
2. Open another window showing the directory the file is in.
3. Right click on the file and select 'copy' from the menu.
4. Move to the first window, right click, and select 'paste'
This should make a file of the original name in the new directory.
If you don't know how to make directories or use them, and if your original file (that you can't open) is in My Documents, then here's some more info.
From My Documents, select File/new/folder. This makes a folder called 'New Folder' and you can leave that name or change it, but you can use this folder for step 1 above.
I think the method from mytwopals would also work.