But the OS does need to be original for you to be safe unless ofcourse you dont mind going to jail for sometime and pay huge fines because of software piracy. But as Dima says OS doesnt need to be original to be safe. I would not agree with that entirely cos in case of windows you wont be able to update it if its not original which means the latest security patches are unavailable leaving your computer open to threats but then again there are other ways to make it secure. M sure there are free security programs which might do a better job than the MS itself to keep ur PC safe.

Regards
Raj