Google Says 5% of Visitors to Its Sites Have Ad Injectors Installed
You can't trust anybody except for open source repositories.The easiest way to get such trash on your computer is installing software from a commercial vendor. Oracle is one major source of headache, if you aren't careful you'll find your 'java' install also gives you a severe case of malware/crapware.There are whole companies dedicated to this concept of piggy-backing junk.