You'll hear a lot of people provide solutions but the only solution imo is to re-format the machine. If you don't have backups then it's the only way to be sure.
If you had a weekly backup with a disk imager like Macrium Reflect or a daily snapshot with a snapshot tool like Comodo Time Machine or Rollback Rx then you'd be able to restore and only lose a few days to a weeks worth of data. Sadly it sounds like this isn't the case, so I'd recommend reformatting and immediately start a regular backup plan.