So..you got the Windows 10 upgrade, huh?
Recently, Windows 10 has been marked as a "Critical Update" for Windows 7/8.x users. This means if you've got Windows setup to automatically update, you're going to automatically be upgraded to Windows 10. (At least until July 29, 2016, when it's not longer a free upgrade.)
This automatic update/upgrade has been referred to as a 'nasty-trick' played by Microsoft.
And if you click that Red X in the upper right-hand corner, they're going to install the upgrade for you, you're not actually cancelling out of it.
But, that's okay. If you got the upgrade, and just really don't want it, there's good news: In the first 30 days after the upgrade, you can roll back to your previous version of Windows! But, let me be clear, you have to roll back in the first 30 days after the upgrade. If you wait longer than that, it's not an option.
How-to-Geek has a good step-by-step on how to Downgrade to Windows 7/8.x on their website.
However, if you're daring enough to keep Windows 10, at least for the first week or two, and try it out, you might find that you like it. It's good a fair number of ways Microsoft is hooking you in the way Apple might. But, overall, the only significant complaint I've heard is the Windows Start Menu is closer to Windows 7, but people just hate the Metro tiles still.
As a result, I'd recommend giving Classic Shell a try. They let you swap out the Windows 10 hybrid Start Menu for the Windows 7 version we're all more used to now. You even have several ways to make that Windows 7 version look.