Thank you for all your input.
The link that DuaneB posted was very helpful and explained it all. Thanks for that.
Thank you Nick for your comment. However in the project that I am doing, the timing of the ISR is very critical and I cannot stop it no matter what. Is there any way around that. Is there a way to let the ISR do its job (read sensors to volatile variables (ints)) and these variables can be read in the main loop? ( I am not writing to them, just read them in main loop)
Thank you,