With extreme care as there is a possibility you could lose the .pst file. The pst file is a huge database file with all your emails in it, avast may have difficulty in extracting a single infected email from within it and putting the file back together without corrupting it.
What version of avast are you using, Home/Pro ?
Some AVs could just see the pst file as infected and not an email within it and delete that or those that can try to extract an individual infected email might corrupt it, the end result being the same a loss of email.
Before doing anything backup your emails, etc. I don't use Outlook so I can't speak from experience if avast can extract infected emails from the pst file, but it can do this from OE's .dbx files.
You could run an on-demand scan either 'Local Drives' or 'Folder Selection' of the location of the pst file, that would include it, but it could be a risk. If you scanned it avast may be able to either give it a clean bill of health or report a virus, if it reported infection I would suggest you make notes of what it found particularly any reference to the actual email and choose No Action. This would allow you to open Outpost and try to find this infection and remove it manually, delete the email then clear the deleted items folder and then compact them (sorry don't know if this is an option in outlook or how to do it). This is I think the safes option, but backup before doing anything.
