I took my computer into Staples to re-install Windows for me. I have my own key on the back of my computer, so they told me it wouldn't be any problem. However, now whenever we log in and log off they system, it shows a counterfeit message. I have tried to validate it, but it won't give me the option to type in my key. It just tries to validate itself online and then tells me it can't and gives me the option of buying a new installation. What can I do?

It is possible to view and change the key that is present on your system. I don't have the instructions myself but I have done it in the past when I needed to, to record the number. Google; how to change XP registration key.

Before you can do that you should have installed the original installer, if not you can not register the key it will just appear invalid to it or (better yet try to install a pirate one ^^ joke)

I would just take it back to Staples. You have paid them to do a job, and obviously they have not done it correctly.