Install Ubuntu without making Windows license void
I think you have been misled by the engineer.
If you really want do dual boot ubuntu with win7, that is a fairly straightforward process.
Ubuntu will happily co-exist in a dual-boot configuration with windows, with windows first to be in place. The opposite scenario is actually the more troublesome to setup. Windows can clobber the MBR+grub setup and render Ubuntu unable to boot but in the scenario you have got, it should not be a problem.
The main question - is do you have sufficient disk space to accomodate the Ubuntu install. You'll have to probably resize your existing Win7 partition to make some space available for ubuntu.
If you run the ubuntu installer, you can do the resize using GParted.
You can also resize the windows partition using windows Disk Management.
After resizing partitions, you should be able to able to smoothly run Ubuntu install.
There shouldn't be any issues with the Win7 license.
As a fallback, you can run a clone image backup of your current disk state which you can restore just in case you need to.
It's possible to resize the Windows partition with Windows still on it. It can go wrong, but it generally doesn't.
Download a legal version if you already have a Windows license. If you install the same version (Home Premium) on the same machine then your Windows key (the xxxx-xxxx-xxxx-xxxx-xxxx code on a sticker on your machine) will be accepted for the reinstall. It's a good idea to have a copy to hand if you are going to resize the Windows partition in-place as, as I said, it can sometimes go wrong.
More generally, what would you do if your Windows install got messed up and you had to install it anyway?
If your sticker with the key has become unreadable due to wear and tear, it's possible to get Windows installations to reveal their keys.
Who told you that? In my experience it was always easier to install Windows first (or keep the pre-installed one) and install linux afterwards. The windows license is not affected anyway.
But it might be that there is some recovery partition from your manufacturer you must get rid of. It should be documented on Dell's site how to do that.