A few years ago, when I was on my degree, my teacher told me that if I make a infinite loop in C it would crash my computer making it to use all processor resources with nothing and I need to reboot my system to make things good again. Today I tested the same situation on my Windows Seven computer and I saw that my computer didn't crashed and my processor resources were just "idle". What changes from 5 years ago until today to change this specific event?
Why a for(;;){} doesn't crash my computer anymore?
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Your teacher told you something that wasn't true to begin with, so it isn't surprising that it doesn't happen.
At most, an infinite loop will make your CPU go to 100% but on any modern operating system other processes will still get time slices and you can easily kill it. An OS would not be of much use if a simple mistake by a programmer made the whole machine hang so easily.
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Multi-core processors are in common use now unlike 8 years ago, which means that a single inflooping process would only tie up a single core nowadays and leave the rest of the cores free to do other work. Even so, you'd have to be running a pretty lousy operating system to allow a single busy looping process to tie up the whole system.
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Windows has had a preemptive multitasker since W95. Even on a single-CPU box, one looper thread would still leave the box useable, (though slower), certainly useable enough to shut down the offending process in the usual way or start the task manager and kill off the process.
To truly bork you box, raise the thread and process priorities to real-time and create as many threads as there are cores, (save your work).
An infinite loop will only "crash" the OS if the OS doesn't support preemptive multitasking. In any decent OS the scheduler will make that process take a break once in a while and allow other stuff to run.
At any rate, if the resource usage is low, look at the generated code - the compiler might have done something smart like optimizing the whole thing away.