Why can't avast scan them?
Just because they can't be scanned doesn't mean that they are infected, there many legitimate reasons for not being able to scan a file.
I wouldn't delete them until I had full confirmed that they aren't required (but being .tmp files they are probably ok to delete), another option is to rename them so if anything was using them it wouldn't be able to find it. After a period of time if there are no adverse effects it should be safe to delete them.
