My question :
1]is Atmel processors are more sensitive to noise than others such as PIC?
no, noise resistance is related to the difference between Vil Max and Vih min together with the input impedance which for all CMOS will be high.
2]I have never seen such instance where our Personal COMputer get hangs on due to any electrical noise ,is that only because it has multi layer boards & isolated supply[smps]?
That and more, see below
3]Is there any 'master solution' [instead of watch dog]on this Problem.
A watchdog is not a "solution" it is "emergency rescue".

I am working on an article, here is what is there so far.

Noise problems aka Strange happenings
Also good basic rules for hardware design

Rules 1 through 4 apply to any design, the remainder apply with dimisishing importance to critical designs, ALL apply to designs for noisy environments.
Noise can show its ugly face in many ways. You may see occsional resets, bad data and who knows what. Beware; however, many “noise” problems I have seen have beeen software bugs.
The easy way to see if a problem is, indeed, noise is to move the circuit to an electrically quiet location and replace all external connections with resistor and LED for output and resistor and switch for input. Noise is particularily a problem in industrial environments and when driving inductive loads.


The cure:
1) Get rid of each and every ground loop.
2) Use a supervisor chip, not the RC reset.
3) Put 100nf capacitors directly across all chips.
4) Put a 4.7 or 10 uF tantalum directly across the '51.
5) Use a 4 layer board with ground and Vcc planes.
6) Use differential serial communications (422/485).
7) Use a good power supply.
8) Put an VDR at raw power input.
9) Opto-isolate all non-differential signals.
10) Put the thing in a steel box (make sure you use one big enough to take care of the heat).
11) Put ferrite beads on all wires at the place where they enter the box.
12) Do not drive capacitive loads (like ribbon cables) directly from Uc outputs, buffer them with TTL or CMOS buffers (140-144)


Have fun,

Erik

Erik