Is it possible to buy microsoft license keys after installing the software?

Jack: Is it possible to buy microsoft license keys after installing the software?
I need to buy a license key for multiple computer that all ready have the microsoft software installed on them by the same cd. Is this possible, how do I do it and how much?

Many thanks.

Answers and Views:

Answer by Linds
Yeah. You do to the store and buy one for each computer. One key per computer. Otherwise call microsoft and they can sell you a key for each computer.

Answer by SimsJunkie
yes.,

Answer by I Don’t Play Facebook
Go to microsoft website for more information

Answer by Jon
Yes, just go to the Microsoft Licensing page and select the appropriate software.