No you can't. Even if you could, no USB would be large enough to hold it, even if you only had a small amount of games.
Civilization 5 is powered through the Steam system so you will need to install Steam and create an account to install and run the game. (See links below)
you can't, sorry it doesn't allow it you can install Linux distributions on usb though :D
In order to install USB device you must have a driver, USB installers can be obtained or included in some motherboard drivers.
Yes, but you must copy ALL of the files from the disc to a USB thumbdrive and install it through a USB drive. If you don't have a USB drive, then you cannot install it.
You need steam to install Portal 2 Why not? It's free
If you have purchased the game on Steam then you need to install Steam on that PC and there should be an install button on the game in your Steam Library. If you have the disk version you just have to put in it and install it using the CD key that came with it.
Yes
No.
You will need to purchase and install an adapter and attach it the the USB connector located on the motherboard.
You cannot install software onto a USB, but you can copy the installer to the flash drive. Then while on another computer, open up the flash drive, and open the installer so it can run on the other computer.
When you plug in the USB device to your computer it will automatically install. If it does not install right away give your computer a few minutes to work on it. If there was a disk included with your device you may have to insert it to install your device.
In the games tab, click "Activate a product on Steam..." to install the game if you have the CD. If you bought the game from Steam, you should see it on your games list. Right click it and install it.