What is a banner-brick?
A banner-brick is caused by a corrupted or invalid banner/icon installed on your Wii-system. Due to the poor error checking of the Wii System Menu, it will see the corrupted or invalid banner/icon as a corrupted system file. When trying to boot the Wii-system, it will freeze after passing the Health & Safety screen or display a message which says that the system files are corrupted.
What pre-cautions can I make against a banner-brick?
It is important that you never install a WAD from an unknown origin. Get your WADs from WiiHacks or GBATemp or any other trusted site/forum. Read the comments, reputations of a site, and you should know if it's a safe WAD or not.