I have a question for you. What do you scan your old emails for? If there are viruses in those emails, those viruses have already speared wide on your computer, since you did open and read those emails.
It is either there are viruses in your computer or there is no virus at all.
Sometime, Windows acts like there are viruses, however, it may be just some bugs in some programs you run that make the system unstable.
Virus that spear by email usually send itself as an attached file to an email. And it will activate when you open the attached file. Antivirus software scan incoming mails for viruses in order to make sure that there is no virus in those emails, and most of them scan new email only since like I explained, there is no use to scan old ones.
However, any antivirus software have the option to scan all files on the hard drive. And of course those old email will be scaned too, since Outlook save those email in a local folder.
Good luck and don't worry too much about virus,