Now, I'm uber-sensitive when it comes to running anti-virus programs and keeping my system up to date so vulnerabilities can be kept in check. But even I get tagged every once in a blue moon (which actually isn't that infrequent, but I digress).
Anyway, my work laptop was infected with a Trojan that resulted in hijacking my browsers (I could only go to Google, and those search results were wacked) and prohibited me from installing anything.
I messed around with the computer for 2 days until I gave-up and wiped my drive and started over with a fresh install.
Following that event, I run Spybot Search & Destroy (freely available here), and have available Malwarebytes (freely available here).
Very frustrating...