Remove (delete) the email client from windows mail. Then readd it.
Open up windows mail,go to tools, accounts, select the account you want to delete, click remove. Close mail.
Reopen windows mail, go to tools, account, add. Then follow prompts as if you were setting it up for the first time. It is good to get the pop3 and smtp settings from your isp for manual setup as well. It will look something like this: email@example.com firstname.lastname@example.org
where the x's is your isp info.
I would also suggest using windows live mail it's a lot better and easyer to use. I used it for a year until I got MS Office outlook 2007. I got it through Ulitimate Office deal. Here are the links. Mail - Windows Live Microsoft Office Discount for US Students -The Ultimate Steal- Office 2007 Software for $59.95