It sounds like the settings for your incoming (Pop3) mail server have become corrupt. It's not uncommon, because your anti-virus software has to temporarily change these settings every time you receive mail. Sometimes the anti-virus software and Outlook don't communicate very well, and things get mixed up.
First, check that you know the address of your Pop3 server. If you get your email through one of the UK's major ISPs the information should be here
http://www.kitz.co.uk/isp/dns.htm
and/or here
http://www.idl.uk.com/email/isp_settings.htm
The following instructions relate to Outlook Express but, as far as I'm aware, they're exactly the same for Outlook:
Go to Tools > Accounts
Click on the 'Mail' tab.
If you've only got one email account it will probably already be highlighted. If not, click on the relevant account, to highlight it.
Click 'Properties'.
Click the 'Servers' tab.
Amend any incorrect information.
Click 'Apply'
Click 'OK'
Chris