A virus attack makes a .PST file corrupt and has the capability of making the MS Outlook behave strangely. In many situations, the MS Outlook automatically sends randomly-generated messages to other users without any notification. There have been several cases where users have witnessed issues like : Windows registries getting edited together with adding, editing and hiding the files or Microsoft Word documents (sent as attachments) getting damaged.
List of viruses which affect a .PST file
