Outlook Express can be configured so that it leaves your mail on the host server, rather than deleting it, when you download it to your PC. This can be useful if, say, you download your mail to your home PC but then want to get the same mail delivered to your office PC. For some strange reason, Outlook Express seems to have switched this option on. Here's how to turn it off:
Go to Tools > Accounts. Click on the 'Mail' tab. If you've only got one email account associated with Outlook Express it will probably already be highlighted. If not, click on the name of the account to highlight it. Click on 'Properties' and select 'Advanced'. Under 'Delivery', you'll see a reference to 'Leave mail on server'. It's likely that there will be a tick in the box next to it. If so, click to remove the tick. Then click 'Apply' and 'OK'.
If you find that there's no tick next to 'Leave mail on server', click to put a tick in. Click 'Apply' and 'OK'. Then repeat the process, as above, to remove the tick again.
Chris