Incessant postmaster message!

wasap Posted messages 9 Status Member -  
wasap Posted messages 9 Status Member -
Hello,
I receive more than twenty messages from the postmaster every day without sending any messages. I suspect that my email address has been hacked. After running antivirus and antispyware tests, the problem still persists. How can I get rid of it?
I would like to format my hard drive, but I don't know how to do it with my setup because F10 doesn't work at startup of Windows, can someone please help me?

Configuration: Windows 7 / Internet Explorer 7.0

8 answers