Save all files in `/Users` to an external hard disk. Do not copy /Library
as it can contain malicious software. Press Command+R while booting.
(source)
Don't do this. Re-install Mac OS X instead.
Now use the – sign to disable all suspicious apps that you'll find. Restart your Mac for the changes to take place.
/Library/LaunchDaemons
/Library/LaunchAgents
and ~/Library/LaunchAgents