Interesting. I wonder whether the file itself is corrupted in some subtle way which confuses Word. That theory goes in-hand with the fact that you were able to copy the file, and that the copy had the exact same symptom.
Do you have access to another machine with Word installed? Are you able to take the file across on a USB stick or CD or network cable and try modifying the same file on the other machine?
If the other machine gives you the same error about the file being "locked and in use", the file is almost certainly corrupted.
Otherwise, if it works on another box, can you go boot your primary machine into safe mode (hold F8 at startup) and try to modify the file from there? There's a possibility that the entity holding your file "in use" is a kernel-mode driver, and that would not show up in Handle or Unlocker. Your anti-virus is a prime candidate. When you boot to safe mode, AV and many other things will not be running, which is why that's an interesting test.
Do you have access to another machine with Word installed? Are you able to take the file across on a USB stick or CD or network cable and try modifying the same file on the other machine?
If the other machine gives you the same error about the file being "locked and in use", the file is almost certainly corrupted.
Otherwise, if it works on another box, can you go boot your primary machine into safe mode (hold F8 at startup) and try to modify the file from there? There's a possibility that the entity holding your file "in use" is a kernel-mode driver, and that would not show up in Handle or Unlocker. Your anti-virus is a prime candidate. When you boot to safe mode, AV and many other things will not be running, which is why that's an interesting test.