It should be safe to delete a temp file hence the .TMP extension.
See if you can delete one to the Recycle Bin.
If you can delete it that means it was created but no longer used/needed.
If you can't delete it then the program that created it is still running in the background and using it.
No temp files should normally be created at the root of the C drive so I would consider one other possibility: your system is infected.
If you are unable to delete it due to file being used or some sharing violation, download the freebie called
Unlocker to see what process/program is locking (using) it.