Windows desktop PCs are the most common type of spam-spewing zombie, but some Windows servers are probably involved too.
Google search - spam zombies windows phishing
Here is a typical article -
The Register - June 2004 - Zombie PCs spew out 80% of spam
Four-fifths of spam now emanates from computers contaminated with Trojan horse infections, according to a study by network management firm Sandvine out this week. Trojans and worms with backdoor components such as Migmaf and SoBig have turned infected Windows PCs into drones in vast networks of compromised zombie PCs.