Outlook Express has a nasty habit of 'losing' the settings which allow you to connect to the incoming and outgoing mail servers. Additionally, your anti-virus software actually changes these settings temporarily, so that it can scan incoming mail for viruses. The anti-virus software sometimes fails to communicate properly with Outlook Express, resulting in the wrong settings being left in place.
First, check that you know the address for your POP3 server. (It should be something like 'pop.virgin.net'). The information will be on your ISP's website but if you're with one of the UK's major providers you can probably get the information directly from here:
http://www.idl.uk.com/email/isp_settings.htm
(Obviously, if you're using an email service which is not provided by your ISP, you need to check the website of the relevant provider, rather than your ISP's website or the link above).
Now open Outlook Express.
Go to Tools > Accounts > Mail (tab)
If you've only got one email account, it will probably already be highlighted. If not, or if you've got more than one account, click on the name of the relevant account to highlight it.
Click 'Properties' and then on the 'Servers' tab.
Check that the correct POP3 server information is entered in the relevant field.
Delete your (asterisked) password and retype it.
Click 'Apply' and then 'OK'
Chris