Installing windows 7 on top af existing windows 7

2 years ago I bought a laptop with preinstalled Windows 7 and a D: partition with installation files.
Now I wanted to 'format' or recover my HD to remove all kind of dirt etc., but I cannot get recovery files for it (and did not make any, when I bought it) - it's a very long story.

So I have bought a NEW Windows 7.

BUT now I am not sure, what happens when I try to install that on top of the existing.

Would it not best to make a good old formatting of the HD first, and how do I do that?

Why would you buy new Win7? You own the license which came with it for the life of the laptop. You can run Factory Recovery from its partition using the hotkey given at boot for Recovery, or clean reinstall as often as you wish.

Clean reinstall using the Product Key on COA sticker on bottom, following these steps to get a perfect reinstall: Reinstalling Windows 7

If you like the current partitioning then leave it. However if Recovery Partition doesn't work anyway, or you want a clean slate, then follow the steps to delete partitions during install and repartition as desired, or just delete them all and click next and Win7 will partition and format one large install partition for you.

Save the Product Key you bought separately to use on another machine so you don't waste the $100+ value of the one which came with the machine and you own for it's life. You can use the installer for this clean reinstall - just follow the steps given.

That's a very, very long story.

But are you saying, that I CAN install my new windows on top of the existing?

Yes. Did you read the steps I gave you?

Some, but I don't understand. What do you mean with "clean reinstall"?

And what is COA sticker?

It cannot be read - scrubbed off

But I owe the pc, not Bill Gates. So can't I get out to some kind of DOS and format the whole thing -and THEN install my new W7