Hi Andrew,
Temporary files (.TMP), are files created to temporarily contain information while a new file is being made. It may be created by computer programs for a variety of purposes; principally when a program cannot allocate enough memory for its tasks, when the program is working on data bigger than the
architecture's address space, or as a primitive form of inter-process communication.
I would suggest you to click on the below link and perform disk cleanup:
Delete files using Disk Cleanup
Note: When you run the disk clean up you may lose some files which are unused for long time and old restore points.
Hope this information helps, please reply with results for further assistance.