To fully answer your questions before these fine folks get off on a tangent and this gets lost somewhere in the mix:
K so I am new to the computer repair business and I have used my windows cd to "make the computer like new" thing. Anyways with people who want windows reinstalled or recovered or repairs or whatever.. Where do you get these CDs? Do you need a new copy of the windows CD? Can you reuse that same windows CD even if I have already installed windows on another computer?
First just want to say that I do agree with the others, the first statement of "make the computer like new" as well as the last question shows a very large lack in knowledge. You say you know more than you come off too, but you should at least know the correct terminology if you have the knowledge. I wouldn't suggest going into 'business' until you have gained enough knowledge and confidence to not have to ask questions like that.
Anyways, to answer your question, you can use an OEM copy of Windows to reinstall the operating system. The OEM version is essentially a stripped down copy (without all the bloated, preinstalled software) of the Windows OS. This allows us tech's to reinstall Windows on a computer and then use the clients valid CD key.
And yes, this is 100% legal as long as you are using a VALID (and the CLIENTS original) CD key to reinstall the operating system on the computer.
I would suggest working on a separate computer before working on someone else's. If you do work on someone else's computer don't get in over your head, if you don't know, ask and research. Also, don't work on someone else's computer that you don't know (only friends and family that know you're a beginner) until you are completely comfortable with doing the work.
There is a ton of good resources on Technibble to take a look at and it's highly recommended that you spend a few hours looking at all that is available here, including a list of OEM downloads.