There is no way to "uninstall" the updates.
I am thinking there are two possibilities:
1. You have some sort of major flaw with your system and should contact Nintendo for repairs (hopefully it is still under warranty). Nintendo is very helpful even if you system is out of warranty.
2. You have a modded Wii, in which case, I and this board have no sympathy for you. Your Wii is ruined. You took the risk and lost. The only solution is to purchase a new (unmodded) one.