Hey guys, this is for the people that have Avast. Avast thinks that LoL is a virus and league won't open. follow these steps:
1) Open Avast
2)Right click on any of the options you want to get rid of, (smart scan, browser clean up, home network security etc) and select virus chest
3) Click Virus chest
4) find the LoL launcher.exe
5) Right click on it and select "restore and add to exclusions"
6) Close it and your client should be working!
Hope this helps with the people that have Avast!