always, when I am sending a mail with an attachment of any graphic file with outlook 2003, an additional file (Named = 7.gif) with advertisement of Viagra is added. This is embarrassing. Avast-screening doesn’t show any virus. Searching with standard tools for 7.gif of windows doesn’t show any result.
Welcome to the forums, Heider 
I suggest you try a scan with malwarebytes antimalware.
Download it from the link below, install it, update it, and then run a quick scan.
Let it delete what it finds.
Post the resulting log here.
http://www.malwarebytes.org/mbam.php