ac load control using a microcontroller via relay
Also, if possible, it would be great to have some sort of isolated circuit to determine when the AC is at around 0 volts (zero crossing) and switch the relay at that point. This would certainly extend the life of your relay and reduce the chances of getting a large electromagnetic pulse from the motor, which may induce spurious signals in the traces (if it's nearby) or the wiring to your microcontroller board. A Snubber is a good idea as stated. Does the problem still occur if you disconnect the motor and just operate the relay?
If so it may be switching spikes from the relay coil itself. Make sure you have some capacitance near the relay to dampen that, and also if possible don't use the same ground plane on your circuit board, for the relay and the microcontroller. Run a separate trace for the relay ground to the power input connector. You may also want to put a diode (e.g. 1N4007 or similar) across the relay coil (reverse biased) to help kill spikes.