how to block unwanted mails

Outlook Express does not have spam filtering. Microsoft offers its solutions here:
http://www.microsoft.com/athome/security/email/fightspam.mspx

Your ISP may have a service available where they filter incoming emails through e.g. Spamassassin.

If you change your mail hosting to e.g. Google Mail, you will have filtering available.

You can also install Mozilla Thunderbird free of charge. It has junk filters and anti-phishing protection:
http://www.mozilla.com/en-US/thunderbird

If you want to know more, this site has information about unwanted emails:
http://www.getsafeonline.org/nqcontent.cfm?a_id=1152