I have an ASUS 900 and had to go through this when I wanted to get rid of the stock Linux that was installed on it. That was over two years ago. Back then, I was using a CDROM and a USB CDROM drive. The installation from a USB-based device is perhaps the only way you can perform an installation of Ubuntu or any other Linux distro.
The trick is to know how to boot from USB. On my 900, you need to press ESC when the first lines of text are displayed. You can then select where to boot from. It should be the same on your 901
I would recommend using some desktop with Ubuntu Linux already installed. From the System menu, there is an option to make a Linux USB startup disk. I can't remember the actual option name. This will make a USB device (or an SD card) that can then be used to boot from . Once started, you will have an option to actually install the Ubuntu Linux.
UNETBOOTIN is another solution to take any ISO and put it on a USB-based storage device so that it will boot from it just as if it was booting from an actual burned CDROM. From there, you can then install Linux. This second option lets you choose among manymore Linux distribution, should you choose to use another one.
On an ASUS 901, I would recommend to use the Ubuntu Netbook 10.10 distribution. You will likely experience a slow display, which can be avoided if you switch to the standard desktop. Installing the full regular desktop distribution is not recommended as it will be too slow for your computer.