Yes, you can delete them.
What version of avast! do you use?
Do you have any big archives on your disk? Did you interrupt/kill the scan recently?
Could you please try to identify these files? They are created as temporary files during the scanning of archives (and normally, they should be deleted automatically, of course). So, could you maybe run another disk scan and watch for these files to appear (similar files will be created and deleted during the progress - but some of them may be left).
Thanks!