Blacksheep - my rootkit problem did not come from a CD or DVD.
I accidentally ran a malicious executable downloaded from the web. Stupid - yes. But at the time I thought my anti-virus and anti-spyware software would deal with the problem. The malicious executable did install a standard trojan which my anti-virus and anti-spyware software dealt with just fine.
However, several weeks later I discovered the rootkit - in this case a spam zombie program pumping out spam messages.
If the rootkit did not get installed when I accidentally ran the malicious executable then the situation is even more worrying because it must have been installed without me doing anything stupid!
Rootkits need not be malicious - but they can be - an the one I had was.