Is my computer a zombie machine?

I always read about giant networks of zombie machines hackers user to mount attacks on different servers or other pieces of internet infrastructure. Like here: http://blog.washingtonpost.com/securityfix/2007/02/internet_survives_major_attack.html?nav=rss_blog How do I tell if my machine is being used for... show more I always read about giant networks of zombie machines hackers user to mount attacks on different servers or other pieces of internet infrastructure. Like here:

http://blog.washingtonpost.com/securityfix/2007/02/internet_survives_major_attack.html?nav=rss_blog

How do I tell if my machine is being used for these nefarious purposes?
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