Outlook 2003-Large Mail-Security Warning

Using Outlook 2003 on XP (SP2) when trying to open a large .pst file (~300 MB as a backup of old e-mails) I got an ?Attachment Security Warning? saying ?The file may contain a virus?.You must save the file to disk before it can be opened. ??.

As the file is already on disc (even if saved again the result is the same) how can I get access to the e-mails on the .pst back up file?