Nothing is 100% proof against infection. You just have to be very careful.
The best removal guide is here: http://www.bleepingcomputer.com/virus-removal/remove-malware-defender-2009 scroll down that page to read it all as the first links are adverts.
It mentions using MalwareBytes (free version is fine) which can be downloaded, updated and run in "Safe Mode with Networking" which should give you internet access in Safe Mode - reached by tapping F8 while booting up.
Update it before running,