Whenever someone hires me to repair a malware-infested computer my first step is to shut it down and boot into Safe Mode. “But Brian,” you say, “I can’t install software in safe mode! What if I have to install AVG, Avast, MalwareBytes, or HijackThis?” Well, do I have a surprise for you!
Symantec, makers of fine security software for as long as I can remember, has a little guide on their website explaining how to enable the Windows Installer Service, and thus enable installation of software, from within Safe Mode and Safe Mode with networking. Check it out!