I personally don't like to give something no limit, although 0 should have worked, did you try to set the chest size to something a little above the file size.
You appear to have used the Win2k Backup see this link
http://web.ukonline.co.uk/cook/backup.htm so a backup that has an infected file when the actual system doesn't appear to have this infection, is a bit of a waste as you shouldn't really restore an infected backup.
If the backup.bkf file is old as well (if 4 months old) then I would tend just to delete it, do a scan of your system and if clean run the backup again, that way you retain the data you have NOW.
Backups that aren't made on a regular basis are better than no backup at all but you need to employ a regular backup strategy, once a month, fortnight, week, etc. I do an image of my hard disk partitions (exact copy) every week and backup volatile data files at least once a day (this includes emails, address book, favourites, etc.).