Its a common problem once you are using the inductive loads in your circuit.Use a free-wheeling diode(an ultra fast diode-like UF5404-is preferred)as near as possible to the relay coil.also you can introduce a filter in the input side of your uC, if the design is not freezed.

and after connecting the load, u may find more spikes while switching the AC loads.there also you have to put a filter of RC type to ensure less noise

alternatively why can't you use a triac to drive the AC load. Zero crossing type Opto-isolator for triacs(MOC3081) will reduce further noise in the AC side as it turns on and turns off only at zero-crossing and also it isolates your controller board from noises

try
saji