MS Outlook allows you to setup multiple pop3 accounts under the same user, so you don't need to log out and log in with a different user. Everytime you send/receive, the outlook fetches all the saved pop3 accounts for this user.
I am not sure if Outlook Express has this feature or not, but I think it is there, you just need to check it.
In MS Outlook, just follow the following steps to create a new pop3 account under the same user:
1. Open Tools
2. Email Accounts
3. You can choose to add a new account from this page or you can continue by clicking next and you will find your pop3 account in the next page with a button at the right saying "Add" which you can click and add a new pop3/http/imap account
4. Proceed with setting up the account normally and insert the required data for it
PS: I am talking about the MS Outlook 2003, but I think the steps will be the same for other versions with very slight differences.