Outlook Express failing

“Either there is no default mail client or the current mail client cannot fulfill the messaging request. Please run Microsoft Outlook and set it as the default mail client.” I have Outlook Express and do not want to change to Outlook. Most incoming e-mail is received, but about once a day or every other day I get the above error pop-up and the message is not received. I have Windows XP,SP2, Internet Explorer 7. I’ve tried Tools/Internet Options/Programs/ changing the preference from the drop down menu from OE to Outlook, back and forth but nothing seems to work and thus I occasionally miss an incoming e-mail. Any help would be much appreciated. Could it be that OE is somehow corrupted?