Unequivocally, the best software for stopping spam is an operating system other than Windows. That's because over 90% of home-based Windows machines (probably including yours) are unwilling zombies and thus the SOURCE of spam that you and I receive. This is fact, not fiction or an anti-Windows orientation. Google yourself for "source of spam" and you'll have plenty of reading material such as http://www.informationweek.com/news/security/showArticle.jhtml?articleID=18200812
Since OS-X requires Apple-branded hardware per the EULA, that leaves a Linux distribution as your best approach to eliminating spam. Also, that solution costs nothing.
To minimize the impact of receiving spam in your personal email - which might be what you meant by your question - use an HTML mail provider on the Internet that bases their servers on Linux. This is much less troublesome than using a local mail program like Outlook or Thunderbird, and hoping your ISP puts a big effort into screening spam while preoccupied with maintaining your Internet traffic flow and providing a help desk for customers.
The two best choices for Linux-based HTML mail on the Internet are Google and Yahoo. In other words, it's best to receive your email only to a @google.com or @yahoo.com account - since they will do the bulk of filtering out of the spam for you. I use both - and my @google.com email receives fewer spams in the actual inbox than @yahoo.com - which allows in about one in thirty obvious spam emails. Everything else I've tried has been much, much worse than Yahoo. Google rocks!
If you continue with Windows knowing you're likely to end up in someone's spam farm, then these four malware tools will work together (requires all four of them) to eliminate malware - and are free. You MUST maintain each package diligently, each week taking at least as much time as installing and learning Linux just once. This way you can run Windows and NOT feed world-wide spam emails every time you connect to the Internet.
Note that the CPU cycles these third party programs take to be effective will decrease your Windows speed and available RAM - and they aren't needed at all for Linux - an OS that already runs faster than Windows.
SpyBot [Malware removal, Registry monitoring]:
http://www.spybot.com/en/mirrors/index.html
SpywareBlaster [Malware payload "Click-attack" prevention]:
http://www.javacoolsoftware.com/spywareblaster.html
AVG Free [classic virus checker - but don't use its spyware tool]:
http://free.avg.com/ww.download-avg-anti-virus-free-edition
ZoneAlarm (Free version) by SoftPoint [interactive trainable firewall]:
http://www.soft82.com/download/windows/zonealarm/
Back to Linux, which is free in every sense. Try the PCLOS distribution (full version) at http://getpclinuxos.com/KDE/ and download the 700 MB ISO file. Then you burn the ISO using ImgBurn [CD/DVD checking and ISO burning] from http://imgburn.com
For instructions on how to add Linux as an alternate OS on your hard drive (so you can still use Windows but hopefully NOT on the Internet), see this article for dual boot [based on PCLOS 2007] at http://www.ehow.com/how_2147183_add-linux-windows-computer.html
Good luck!