Hello Kylie,
in the system.log I don’t see anything suspicious.
I wouldn’t delete a backup file directly - I would not be sure, how a backup tool would behave in a moment when you would try to do a restore. For the Time Machine, if you still have an original infected file on your hard drive, see attached a guide how to remove a file from all backups: http://reviews.cnet.com/8301-13727_7-57597578-263/how-to-clear-unwanted-files-from-time-machine-backups/
As the virus was in yours mailbox, the infected file will be downloaded from a mail server to your hard drive again, until you delete the mail message that has it as an attachement. If the infected file is only in backups and nowhere else, I would leave it there, it is harmless.
Best Regards,
Martin