One common source of malware is free application downloads.
Download sites used to be a great source of free software, but they are not safe any more.
This article investigates the biggest download sites and finds that they all bundle the downloads with adware, scareware and worse. A simple freeware download can render your PC unusable!
Once these sites get their hooks into your system you will experience things like reduced performance, hijacked search results, injected ads, crashes. Most worryingly, there is no way of knowing what data they may be capturing, whether it be passwords or documents.
Unfortunately, anti-virus software often lets through the malware that these sites deliver.
It can be a significant cost to repair the damage done by such malware, so prevention is the best option.