previous OS be preinstalled?
or you install it?
Its possible that either at the store you bought the OS at, or a visitor to your room, stole the key from your disk and used it to install a copy. That means since only one key can be used at any time yours wouls show up as already taken. It wouldn't be the first time a store employee or customer opened the package and stole the key. It does happen.
If you have all the original packaging, contact Microsoft and explain your situation and offer to fax them copies of your purchase and packaging.
previous OS be preinstalled?
or you install it?