Internet Security

I just wonder if someone might be able to help me,The thing is i bought a Internet Security of eBay and installed it on my PC,but 2 weeks ago the PC broke beyond repair so yesterday i bought a replacement,But the Internet Security was for 1 PC only so i unable to use the Serial Code again,Could someone explain to me how i cancel the Internet Security my old PC,and use it again on my New PC <_< ,Many thanks.