I have OEM copies for all version of windows that I've purchased at some point. It's perfectly legal to use these as installers for client PCs.
If a client has a valid licence attached to their PC then there is no problem using that disk for installs.
If however, the client has no license key, then Iwill ask them to purchase a new copy.. Simple as that.
Most PB and Dell PCs have rescue partitions any how so they don't tend to be a problem, providing the the HDD hasn't blown.
As A PC technician, it's important to stay on the right side of the law at all times, with no exceptions.