Many malware infections begin online with pop-ups which pose as virus scanners.  They install malicious files and change system instructions to mess up your machine with automatic web page redirects, false virus warnings, and more.  I carry with me some of the most advanced pro-level system analyzing, scanning, and cleaning tools available.  I use my own systematic technique which succeeds without data loss, file damage or system changes where others may tell you a reformat/reinstall is necessary.  Usually I can completely repair even the worst infection during a house-call which is the same rate as a pickup & return.