Highlighting's a long process, much like deleting them individually. It's unlikely they're classed as system files as they're four months worth of error reports on an issue that I resolved a while back.
I've already tried Disk Cleanup but it didn't touch them.
I'm beginning to think the easiest way may be to revert to good old DOS and delete them that way.
You say "I've already tried Disk Cleanup but it didn't touch them." Maybe you only ran the first stage of Disk Cleanup. You need to run stage two. To do this click on "Clean up system files" when stage one has finished and it will run a second time.