I am having problems with Mozilla Thunderbird - prompss for username & password - since I added two O2 accounts. I can get my maul using Outlook 2003 but I would like to try Outlook Express.
I have done a few searches bu so far cannot find out how to get it.
Where can I get a download of Outlook Express?

you cannot download outlook express. It is copyrighted material and you need to purchase it. Thunderbird is far better in my opinion and it is free. You need to configure TB - it's easy. If it is asking you for a password, enter the password.

When I open Thunderbird & click "Get Mail" messages for my old ntlworld account are delivered to my inbox. No messages for my new O2 account are received. The error message "Sending the password did not succeed.Mail server mail.o2.co.uk responded : invalid user or password."
When I click ok I am prompted to enter password again. I enter the same password that I use to log in to my account on the O2 website & get the same message as above. I have two user names with O2 - both have the same password. I am prompted for password for each username one after the other. I am ticking the box "Use Password Manager to remember this password".
I have looked at passwords stored in Password manager & they all seem correct.

You don't need to download Outlook Express. If you have Windows OS, Outlook Express is already installed as a Windows Component. If you would like to set it up for use as the default mail program, here is the easiest way:

Right click on Start> Explore> Programs> Outlook Express> find the msimn.exe file on the right screen and double-click to open:
Once open, you will need to put your ISP server in here:
Open OE> Tools> Accounts> Mail> Properties> Server tab>> enter the POP3 and SMTP servers for your ISP, Account name and password (phone call to ISP will do it).

Once done: Open IE> Tools> Internet Options> Programs tab> set Outlook Express as the default email. Check 'IE should check to see if it is the default email'> Check now> answer Yes

When it has been set up, it's ready to use. It's already on the computer so downloading isn't needed.

If you would like to create a shortcut for OE to put in the Quick Launch Toolbar:
When you're in windows explorer> right click on msimn.exe> Send to> Desktop to create a shortcut.
Right click on the Taskbar> uncheck 'lock the Taskbar'> drag the OE shortcut into QL> re-lock the Taskbar.

then your account information for the new account is incorrect. This is a configuration issue. You will need to contact the administrator to get the correct configuration data. changing email client software will not change the problem.