of the computer to see about obtaining a replacement installation CD which they can often do for a nominal cost.
Alternatively you could contact Microsoft via the following link about obtaning media.
How to replace Microsoft software or hardware, order service packs and product upgrades, and replace product manuals
If Neither of the above methods work (OEM may no longer support the specific model you have) you could purchase an OEM installation CD and after installing the OS you make use of something such as Belarc Advisor to see what hardware you have and locate and download then install all the appropriate drivers.
Here is a link to one legitimate source of both OEM and retail OS media (there are many others if you want to search with google for OEM operating system but this is a source I have used in the past quite often) -
My school was recently given a number of nice computers from a business that was replacing them. However, they wiped the hard drive clean so the OS is gone. I have the serial numbers for the XP and thought I might be able to download or purchase a CD as a replacement for the original OS. Two questions - is it possible? and where do I find what I need?