I rather think jack is actually using Outlook - not Outlook Express:-
1) because he says so
2) because this is a known fault with Outlook up to Outlook 2002 (XP):
Jack - have a look at this articlehttp://office.microsoft.com/en-us/outlook/HP010210641033.aspx
there are other solutions, too, so, if this does not work, have a poke about on the web. We have had this problem several times, and it usually occurs when the Personal Folders file gets near 2 Gb (about 1.8 Gb seems to be the danger point).
Once you have - hopefully - recovered your folder, check the sizes and if necessary make a second Personal Folders folder so as to keep the size down;
get a later version of Outlook - 2003 or 2007. The problem does not occur in these two latest versions since the structure of of the Personal Folders (.pst) is different: it uses UTF-8 encoding.
Hope you have some joy with this.