hi shahzad ,
i've been keeping track of your postings ,i had a similar problem a long time a ago and all the above suggested methods would'nt work due to some reason ,anyway i solved my problem by using a seperate 5V winding and a 12V winding on my transformer and to transfer signals from the microcontroller )i.e 5V logic) to the the outside world (i.e 12V logic) i used an optocoupler (4N35 should do very well)
this approach does two things firstly it isolates your sensitive microcontroller signals and secondly prevents to a large exent spiking received on account of your 12V logic (relay activation and so on...) , you should have no problems in designing the opto-circuitry.if any let us know.--all the best