Checked all my usernames in IIS, all seemed fine. However, to double check, reset the password for the account I was using and re-ran the OWAUser.wsf script on the archiving server.
Synchronised all archiving tasks and restarted the admin service, still the same error. "The Remote server returned an error: (404) Not Found".
Strange thing is, error message changes depending upon where exactly in the Yellow, archiving preview link box I click! If I click exactly on the part which says "click here to preview original item", I get a windows logon box, I enter in my details and I get a standard HTML page cannot be not found" page. But if I click anywhere else, I get a windows pop up error message (as mentioned above).
I've logged a call with Symantec now, had one reply asking me to send some logs to them, did this two weeks ago but still waiting. Hopefully this can be sorted. Currently holding up my entire Exchange 2003 roll out.