Could be tough to track down. You could clear all your cookies from your browsers then make sure you have Windows Auto-updates turned off. You probably already ran CCleaner..... if not, try that also as it has a registry fix utility also.
If it still persists, you could install a learning software firewall such as Comodo that will report/prompt you for all traffic. Change your wifi password and make sure other applications aren't attempting to update on their own. Monitor your processes in task manager.
Also, in LoL..... I believe there is a check-box somewhere in the load screen options that limits or negates some kind of other traffic they want to barrow from you.