Windows 7: Upgrade question: win7 home-pre to pro?

Hello,

I just bought a new pc and there was something wrong with the hard-disc.
So I went to the shop and they fixed it.

It was a pc without windows installed and I have a Windows 7 professional disc. In the store they installed Windows 7 home-pre x64 to check if it all worked.



Now, I want my Windows 7 pro installed, without losing the files and programmes which are on there already. (My brother installed everything without me knowing lol).

Anyway how do I do this? I know about the windows upgrade system but they keep talking about a "upgrade key"..... is an upgrade key the same as the serial code?

Im just looking for a way to get this done without having to make a backup etc.

Big thanks in advance!

Take a look at this:
Windows Anytime Upgrade - Upgrade your Windows 7 edition - Microsoft Windows

No install files needed.
Only the Upgrade key is need.
Anytime upgrade only adds-on the Programs & Features from version to version.
Windows Anytime Upgrade - How to