the program works fine on proteus (lab center) but doesn't work on a real board ???!!!!!

I am using the ADC ... to do PWM... according to the comparator o/p
What ARE you doing???? using the ADC to do PWM??? never heard of that - then you start talking about a comparator, you need be more clear.

You need to separate the parts to check your hardware. The below assumes (bad, I know) that you are compating some analog input to some fixed value.

does the input to the comparator
1) go to the right pin
2) look right on the scope
a common mistake we all have made:
3) is the pin actually comparator input
4) is the pin correctly configured (most simulators ignore such)
5) is your reference voltage correct

Erik