Just a quick warning to anyone still using Outlook Express in Windows XP: Do not allow any of your mailbox folders (Inbox, Sent, Trash, etc) to exceed 2GB in size. At best it renders you unable to modify the contents of that box, which can halt your mail flow if it’s your inbox. At worst, it prevents Outlook Express from launching correct and in doing so prevents you from fixing the problem.
To see how big your folders currently are, you’ll have to navigate to C:\Documents and Settings\<your username>\Local Settings\Application Data\Identities\<random string of letters and numbers>\Microsoft\Outlook Express and look at the various .dbx files.
If you find yourself in a position where Outlook won’t launch at all, you can take the offending .dbx file and rename it. A 2GB Inbox.dbx file for instance can be renamed to something like Inbox.dbx.bad. The next time you launch Outlook Express, it will create a new empty Inbox and begin fetching messages again. Getting access to the contents of Inbox.dbx.bad is much trickier, but at least you’ll be receiving mail again.