Mystery File Eating By System Partition

Severl weeks ago, I got a warning that there was no more space on C:,my system partition. Properties showed 0 free space. I tracked down the problem to a file named mnc82DB.tmp, which was about 50GB. It was in the folder usersownerappdatalocal emp. I have no idea where it came from and Google has no entries if I search on that file name.

Trying to delete it fails, saying that it is open in another program.

It's very annoying sice if I try to print something I will lose if I reboot, it won't do
it (0 free space). Another weirdness is that if I try to type something into the Firefox Goggle engine, the letters are entered in reverse order.