I have suffered a similar problem in the past – I think that Outlook Express locks up once the memory usage exceeds a preset value. When this happened to me, Outlook Express would not even let me delete items from my mail box (whether on or off line).
I solved the problem using the solution offered by sara3 (downloaded the mail via a website). My PC would then allow me to delete items from my in box – and things worked fine. I now regularly ensure that my inbox does not get too big, by deleting junk mail.