- Network - Computer worms can hop to other machines through the networks.
- Hacked Websites - Hackers use compromised web sites to distribute malware.
- Ads - AdWare is kind of malware which is distributed through Advertisements in the background.
- Free Software - We cannot say there is no backdoor embeded into the free software.
- Phishing Attacks - Easily convince users to download the malware.
If there are any other, please fill in.