Outlook Express has a nasty habit of 'losing' your account details. Go to Tools > Accounts. Click on the 'Mail' tab. If you've got more than one account, click on the relevant one to highlight it. (Otherwise it should already be highlighted). Click on 'Properties', then on the 'Servers' tab.
Check that both incoming (POP3) and outgoing (SMTP) servers show 'mail.virgin.net'. Then check that the account name shows the first part of your email address. (i.e. the bit before the @ symbol). Delete and re-enter your password. Ensure that 'Remember password' is ticked but that 'Secure Password Authentication' is NOT ticked. Click 'Apply' and 'OK'.
If that doesn't work, try deleting the account completely, and then setting it up again. Start as above but click 'Remove' instead of 'Properties'. Follow these instructions to re-create the account:
http://www.virgin.net/helpme/email/outlook6/1. html
Chris