How do you know if your computer is being used as a spambot?

Answer 1

Your computer starts acting sluggishly and if you're on a messenger type program it'll spam people that you have added as friends.

Saying stuff like photo of me :"fake link that leads to a auto-downloading virus"

To fix:

Change your password for messenger based products

get a decent anti-virus and clean all problems found..

Don't be foolish and download 1001 things you find on the internet.

Answer 2

Actually, the computer itself typically doesn't have spam. It is your email site or email client that has spam.

If you are receiving spam, you would know it. In your mail, you'd find the same things over and over, very often, the words in the email will be misspelled (deliberately to avoid filters). The mail will be about things you don't care about, and may be for people of the other gender. Typical products that get spammed are "diploma mills" (fake schools), male enhancement products, prescription medications (often counterfeit ones), breast enhancement pills (which usually don't work), weight loss products, investment opportunities, adult materials, various websites, and sometimes ideological spam (eg., it is for or against abortion, homosexuality, ethnic minorities, etc., or it is promoting a religion). There are many other types of products spammed, but those are among the most common ones.

On rare occasion, you may get popup spam. You might get random messages as soon as you turn on your computer, or your web searches may get hijacked. Those can be symptoms of malware, and you should use a scanner to remove it.