Ubuntu for a Dell D630 laptop with no Windows
Ubuntu is a linux operating system, just like Windows is a operating system. You do not need windows installed to use Ubuntu.
All you need to simply get started is a live DVD or bootable USB and choose to boot from it when you turn the laptop on.
It's a simple walk through and choices from there on out. You can try Ubuntu live and then install it from there or install it directly from the supplied boot options.