I also had this happen 'live', one of the major problems as I recall was that Windows XP (and earlier) had the bad habit of enabling a ton of internet facing se

I also had this happen 'live', one of the major problems as I recall was that Windows XP (and earlier) had the bad habit of enabling a ton of internet facing services by default, and these services contained lots of vulnerabilities so you didn't even have to manually 'go online' using a browser or some such in order to be infected.