most likely it doesnt have an operating system on it
you can download a 30 day trial version of windows 7 off the internet.
i would sugest trying this or using your install disk from windows
if it installs and boot's. you need to go and buy a copy of windows or buy a license key online from microsoft
the keys cost about the same as a new copy of windows with a disk.
you may want to consider a retail copy over an oem version because they can easily be transferd to a new pc
(an oem version is only £20-30 cheaper but windows 8 looks rubish and most people will stick to 7 till windows 9 comes out. which probably wont be for another 4-5 years now