It depends usually. If you bought the computer from a manufacturer such as Dell, Acer, or HP, they will attach a small sticker that looks like
this. Its on the bottom or side of the machine and depending on the type of license you bought, can be used again. Otherwise if you personally purchased a CD, it will come with a serial inside of the packaging so you need to find it. The actual recovery CD is just a copy of whatever operating system you bought with the machine sometimes with other tools on it for recovering information or tinkering with a rescue partition if one exists, and has no activation information. If you don't have a sticker and don't remember ever purchasing Windows anywhere, you might want to consider alternatives or go buy a license. If you want to save cash and find the recovery disk with the operating system you want, you can just buy a digital license and install the operating system from the recovery cd and everything will update through Windows update fine.
Hope this helps.