-Install the latest Windows service pack (SP3 for XP, SP2 for vista)
-If you're using Vista or Windows 7, install League of Legends outside of the Program Files directory
-Right clicking the launcher and running League of Legends as a windows admin. Sometimes you have to make a new admin account and create a password for that account, before right clicking the launcher and selecting "Run as..."
Go here C:\Riot Games\League of Legends\game then right click Heropak_client, and right click restore previous versions, IF YOU'VE ADDED CUSTOM SKINS and are experiencing the 10% 27/28 launcher crash C:\Riot
Games\League of Legends\game
"I uninstalled and reinstalled the game (ran the installer, selected the repair option, then ran it again, selected the uninstall option, then ran it again, and selected install) and it has now resolved the issue for me."