I had a slight problem with my computer but got it fixed.

Now when I go into (tools, internet options) and run the (smart setup wizard)
with the correct e-mail address, it then shows under (receiving (POP3)) my
address with a space before and after the @ sign.

I have had people send me test messages using the address without the
spaces and I can receive them but only by selecting the address with the
spaces on the Selective mail download screen.

If I select the address without the spaces it tells me that the username or
password is wrong.

This is no big problem but I would like to know why it does this

Can you post screenshots of your settings?

I suspect that you have two POP3 host definitions configured, one whose name is your email address with the spaces and the other one without the spaces. Go to Tools > Internet options > Receiving (POP3) tab to see if both are listed. If they are, you know that the one without the spaces is configured with an incorrect credential so probably should be deleted but you might first want to rename it using the Edit button and then disable it. Now you can rename the other one to remove the spaces.