How To Uninstall Windows Vista On Trial Run Off My Computer So I Can Install Microsoft Office Enterprise?

I bought a laptop and it automatically had Windows Vista Home basic which has now expired. So all I can do now is go on the internet in order to download another Microsoft package off the internet. But I bought software for Microsoft Office Enterprise 2007 and I can't get it on. I think my computer expects me to download Vista from the web but I've already bought more software for Microsoft Office Enterprise 2007. Does anyone know how I can uninstall Vista in order to install Microsoft Office Enterprise? Ifyou need more details please ask for more.