Where to find product key for windows xp sp3?

First I want to tell you that if your computer is infected with virus then you need to scan your system with anti virus because virus can even create problem with its functioning and after that you may get same speed of working and your system does not have any anti virus then you should definitely use some good anti virus.

Next things is that every genuine windows Operating System have different and unique product key which is provided along with it. You just need to find your product key nicely. And if you are using pirated version then you must have forgot to note down your product key which you have to ask the one from whom you have copied and i hope that he can provide you the product key but using pirated version is not good for your system ( in case if you are using pirated version) because while updation your system may be caught by windows and after that windows can take a legal action against you as piracy is not allowed. And if you have the genuine copy but still you can't find it, then you can consult windows support by navigating through windows.com and launch your complain or you can even move to the place from where you have bought your software copy they can help you to an extent in a best way.