garbled screen is normal and can happen on manny apps not just priiloader.
To check if you the installation succeded, turn off the wii (hold down power on the console till it turns off to ref light) turn on while holding down reset button. If instead MMM loads (you have bootmii/boot2 which is good) just unplug your sd card for a sec until your are in priiloader, replug the card. and continue with the guide you are using, i hope it is mauis softmod guide if not, stop using youtube to softmod and stick to wiihacks super guides.
If priiloader does not show up, run the installer again and make sure you hold down the B button when pressing + to install priiloader, do the same from above.
(further question go in the guides thread)