Most likely some vulnerability in some program you're using. Common culprits, often with known unpatched vulnerabilities: Windows, Internet Explorer, all Adobe Software (Acrobad, Flash), Windows Media Player, other media players, even virus scanners and firewalls.Also, I am unsure how on earth did the viral jerks nest inside of my PC
You should spend an hour every day to keep your system fully up-to-date, it's a very time consuming task on Windows, no central update functionality. There exist some special tools for Windows to check all installed programs but I do not know how reliable those are.