what i understand from your statement, your flow is, sent sms to pc, pc send signal to microcontroller, microcontroller trigger the light via relay. if it was what u mean, actually you can directly trigger the light just by using GSM modem and microcontroller. But if you still need to involve PC in this case, why dont you try this, Your PC send sms using GSM modem, then at the receiver side you can use one more GSM modem to trigger microcontroller then activate the light. it just my suggestion, but the cost will be higher then what you think before.