I would try to contact the people you bought the computer from first.
They would have that answer for you quickly.
However, it is possible that they are illegally providing a Microsoft
product to you.
or
Sometimes, on the hard drive, there is a folder where the setup files are
stored.
That saves the hardware company money by not providing CDs to you.
Look for a SETUP folder, or SOFTWARE, or be creative and look around.
If it's a brand name, look for the name: \DELL \HP, etc...
No one here is going to give you a CD-Key or tell you how to illegally
download...
-Joe