Ok, so normally when a computer boots, it does this:
1) FIRST it loads the BIOS.
2) SECOND it loads an operating system from the HARD-DRIVE.
Different computer manufacturers have different settings in the bios by default, which you can change.
So to change the boot-order on your first laptop you can try the following:
A) CHANGE BIOS BOOT ORDER!
Go into your BIOS settings, and tell the BIOS to FIRST check for a bootable USB-flash-drive, BEFORE it loads the operating system on the hard-drive.
How do you get into your bios? Well, when your first turn on your computer, you will have to press a key at start up. For example, with my computer, I can press either "F2" or the "DEL" (delete) key when I turn on the power, and that will take me into the BIOS.
When I turn on the power, I simply just keep pressing that key over and over, until it "catches" and I enter the BIOS.
Confusingly, different computers have different "BIOS" access keys. For some you have to press "ESC"... others you press "DEL", or "F2" or "F12" or "ENTER", etc... Usually the computer will flash a quick-start-up screen, and that screen tells you which key to press.
The problem is that screen sometimes flashes by too fast, for you to read it!
If that's the case, you can just film the screen at start-up with your smart phone, and then play back the video on your phone, and pause it, so you can read what's on the screen.
Another way to find out is to try googling the model/brand of your computer followed by the words "BIOS key", and you should get your answer there as well.
Once you get inside the BIOS you have to search around all the menus, and play with the right settings to change the boot-order. (All the BIOS' are different, and some are very confusing... but if you spend a bit of time in there, you'll find the right settings.)
NOTE:
Getting into the bios, to change settings, usually has NOTHING to do with the operating system (Ubuntu or otherwise). So the fact that your hard-drive has Ubuntu installed should be IRRELEVANT. What is relevant is simply the key you press (to access the BIOS) when you first turn on your computer BEFORE the operating system loads.
ALSO NOTE:
The reason your second laptop is able to boot the USB, might simply be because it has a different default boot-order (in that it first checks for a bootable USB, and then if it doesn't find one, it then checks the hard-drive). If somebody else installed Windows on your 2nd lap-top, for you in the past, then that person might have been the one to change the boot-order! (Or maybe the manufacturer set the boot-order in that way...)
B) UEFI BIOS
So yes, with most computers Method A above will work, and allow you to change the boot order.
BUT... very ANNOYINGLY some of the newer computers (particularly those with UEFI bios) LOCK themselves to a single operating system. This is often called "SECURE BOOT".
So no matter if you go into the BIOS, and simply change the boot order or not, it will ONLY boot into the operating system that it is "locked onto".
Thus with the UEFI bios, you often have to take some extra steps, and hunt around the settings in the BIOS menu, looking for other options to "turn off "SECURE BOOT", and/or change the default boot operating system, and then finally... change the boot order.
So in other words, if you have a newer UEFI BIOS that is "locked" to a particular OS, you have to go into the BIOS, turn off secure-boot, and then perhaps change the name of the operating system it is pointing to, and then finally also change the boot order.
Either way, to boot from your USB, the key is to go into the BIOS and change settings there.
AFTER ALL THIS... if you try everything and are unable to do it, you might be able to pay a local consultant or tech support person in your area (or even take your machine to Geek-Squad at Best-Buy) for them to have a quick look at, and simply show you how to change the boot order on that particular model.
They should NOT charge you very much money, just to simply teach you to change boot order... so it shouldn't be expensive. Once they show you how to do that, then you can just simply install the Windows-7-OS yourself, as you are planning to do.
