It depends on the brand of PC what can be done, Dell, you can use any Dell re-installation CD as long as it is the same version as the COA key, on any Dell. Then download any software and drivers for that model from Dell.
HP does not have a re-installation CD, they have what are called Recovery CD/DVDs, and newer PCs are tied to model in the bios, so the recovery set will only work on the intended model. Otherwise you will have to use a XP re-installation CD and then use the COA key to install it.
Over the years all manufacturers have changed how they restore a PC and type of media they provide for doing it, so the question can be complicated depending on Make and year of production.
All that being said, it requires you to find the proper CD/DVDs to reinstall the same OS that came with the PC and use the license key on the COA sticker (only needed if you use media other than what the manufacturer supplied) that is on the PC somewhere.
Being a PC shop, you do not want to do anything illegal like pirated keys or software cracks.
When reinstalling from a re-installtion CD/DVD on a Dell, HP etc, it will not be like a factory install and is a lengthy process to get the job done.
There is no easy way when the recovery partition is gone or never existed or you don't have the proper Recovery media.
Recovery media is different than a regular Operating system CD, recovery media will restore the PC to as shipped condition with all software and drivers, a re-installation CD will not.