I'm badly suffering of a stupid behavior of the PC whenever I press STOP button in Mach3 during curving something. PC just gets rebooted by the stop command in the middle of the job (running). It's a bit freaky to explain because it doesn't happen in regular stop.

For instance, I loaded a tap file to cut something and pressed the START button. Mach3 is doing what it supposed to do and I can stop it anytime without any problem. But I have noticed that my PC gets rebooted if press the STOP button somewhere in the middle (or near to finish) of the job due to an urgent reason or to stop the test run. It doesn't happen if Mach stopped in the begining of the run.

I tried two different PC's and it was the same behavior. For your information, in both cases it was DELL Optiplex 260 and there was just the Mach3 and no other softwares were installed.

Please help me to get rid of this problem. I don't really understand whether it is problem in the PC's or some settings somewhere.

First thing to say is Stop should only be used after motion has stopped or you will likely lose steps, so it is only in an emergency situation you should use it. The proper procedure is to press feedhold then after motion has stopped you can safely press Stop if you wish to edit code etc.

Try the optimisation steps detailed in the document on the Mach download page.
Hood