|You haven't provided a specific enough model:|
If you're not sure which one you have, find the Product number on the label on the outside of the case, use the part of it before #, if that's in it, to search with.
Once you use the right thing to search with, click on the Download drivers and software, or similar, link.
If you don't have Recovery disks, you will need a Windows disk.
When you run 2000 and up's CD by booting the computer with it, you can delete the existing partition(s) and make a new one, then run Setup.