Wow, everyone wants to get into the PC repair business. :)
To answer your question, most OEM PC's will have the OS already on a System Restore CD or even on a System Recovery partition on the hard drive. Either one should work. I would recommend using those since they'll contain the specific drivers that are needed for that particular PC. Otherwise, if you use a store bought CD, you'll have to go hunting around on that companies website to find all the various downloads. I've found that these companies don't always put ALL drivers/software on their websites.
In my experience, I've installed Win XP from a store bought CD on to various PC's and just used the Key that's located on that PC. The versions are usually the same. Very few OEM's will provide an actual Win XP CD. Dell is one of the few that I know does, but this was for their business systems. I did use that Dell Win XP CD to install Win XP on an HP using that HP's Key. It worked, where as the System Recovery for that HP failed since there was a problem with the motherboard. (The on board NIC had died so any reinstall failed with a "Code Purple" which is because of a modified Win XP to prevent it from being installed on other computers. The problem here is, if components fail in that computer, you have to send it in to be fixed before you can get a working system again)
You may also want to look a the following links as they may answer some other questions you might have. You're not the first to mention you wanted to do PC repairs and ask for help. :)
My PC Business
Questions about starting my own repair business?
My Computer Repair Business Plan (comments and opinions wanted)