Michael wrote ..

"2) Alternately he is trying to say that his microcontroller is being upset and crashing or operating in an unpredictable way whenever some appliance is being connected to or removed from the power line."

Yes that is the correct assumption.

" or .. filtering the power and ground of the microcontroller board."

And therein lies the solution.

Note for Kurnia : You said that even plugging / unplugging a soldering iron is upsetting things. Now that is bad - so you have to try out the ides given by the members to filter out 'that' spike that enters your MCU supply.

And when you have crossed this stage go for this acid test :

1. Let your MCU operate normally.
2. In the same power outlet that feeds your MCU power supply, try plugging in/out one of these : A universal drilling m/c or grinder ( the more the brushes are worn out the better !) , a PC monitor or TV, a choke started flourescent lamp.
3. Your MCU should still work without crashing.

Good luck.

Raghu