It is rarely necessary for a domestic user to buy software to defeat spam.
Any half decent ISP should have spam options on their mail server (BTYahoo is very easy to use) and if any get through that, any proper email client (Outlook) will also filter it for you when set correctly.
Filters on ISP mail servers and email clients are active filters. In other words, once you get something you consider spam, you can block any further traffic from that email address simply by adding it to the spam list.
Anti spam software (which big businesses use) usually has a passive component, which examines the headers and contents of all emails and blocks anything the rules don't allow. The trouble with the domestic user buying this stuff is that to be any good it costs a lot of money. Cheap stuff is little better than using the previously mentioned server/client options. If your ISP does not have them...maybe time to change??
EX IT geek out.