To directly answer your question, yes, if you buy an OEM version of the same edition of Windows 7 (you cannot move from Home to Pro or vice versa) then you will be able to re-install Windows. Dell are pretty good at offering all the drivers for their laptops on their website too. However, you don't have to buy an OEM version of Windows 7 from Dell. If you can get hold of the retail version of the same edition of Windows 7 (and there are multiple ways to do this) then you can edit a file on the disk to turn it into an OEM version - which will then happily install and register using the product key attached to your laptop. Full instructions to do this can be found.
Before you begin • Make sure you have: • Your Windows product key (xxxxx-xxxxx-xxxxx-xxxxx-xxxxx).. • An internet connection (internet service provider fees may apply). • Sufficient data storage available on a computer, USB or external drive for the download. • A blank USB or DVD (and DVD burner) with at least 4 GB of space if you want to create media. We recommend using a blank USB or blank DVD, because any content on it will be deleted.