browser addons - How are chrome extensions installed without my knowledge?

I opened Chrome this morning and discovered the red AdBlock icon near the omnibox wasn't there. I opened the Extension list and discovered two extensions I'd never seen before, and all the others deleted (Herpderp youtube, adblock, etc.)

In a panic I deleted the unrecognized ones that had (IMO) obvious malware names, but in my haste I dind't write them down or anything. (I guess I'm not that super of a user :( ) So, I'm not sure which they were, but I certainly didn't install them. If they appear again, I'll edit this post with their names.

How can extensions be installed without my permission or knowledge? And, how can I prevent that from happening?

EDIT: That was fast. These two have been installed: DOOwNSaave 5.2 and FindBeSStDeAl. Where can they come from?

Extensions can be installed in to web browsers by other software on your machine, although most browsers (I believe Chrome included) will inform you when this happens. You've got either a malware issue or an adware issue (potentially dropped by the installer of some free software, you know the type). You can try running a standard virus scanner on your machine (Microsoft Security Essentials, Avast, what have you) but be aware that any antivirus running on the compromised machine may also be compromised - clever malware will prevent the scan being effective. Your best bet is to run some type of offline scanner (Kaspersky produces a free boot disk, for example) on the machine.