A simple trick I used to use to see if the spam source is a hacked "bot", ie. the compromised computer owned by some sucker, is to try to connect to it using simple windows file sharing. It's simple:
file://theipaddresshere/c$
There are other ways to access the admin shares, but if this one doesn't work then it's unlikely the others will either. If it's password protected, there are plenty of trivial dictionary attacks out there that can try a few thousand "typical" username/password combos.
If you can get in, chances are it's a hacked computer because no self-respecting "real" hacker or spammer will leave file sharing open.
Before this sort of thing was raised to a felony offense, I used to do a virus scan and run ad-aware on their shared drive (which took freaking forever because of the terribly inefficient file sharing protocols) and clean out anything I found. Then I'd switch their windows desktop background to a bmp file with big red text "your computer has been hacked - turn off file and print sharing, get a firewall, and run anti-virus software!". If they had a shared printer, I'd send the same bmp to their printer. Sometimes to be extra goofy if they were running win98, I'd turn echo on in their autoexec.bat, echo the same text message to their screen, and enter a "pause" command which would force them to hit enter to continue to boot.
I suppose I had too much time on my hands, but I gave about a dozen obviously compromised systems this treatment before it became a felony offense and I stopped.