1) The greatest advantage IMO is that you can make a 'live USB/DVD' from any Ubuntu release, and use it without installing it on your HDD. This feature can prove very useful to access your data when the main OS refuses to start.
2) It boots up much faster than Windows.
3) You don't need an antivirus for Ubuntu since it has built-in protection from viruses and malware (this is true for most Linux distributions), and Linux is much more secure than Windows.
4) The user interface ('Unity' by default) is clean and uncluttered. (Not everyone likes that interface though! But it can be changed.)
5) It can run on a machine with as low as 256 MB of RAM. The Xubuntu and Lubuntu variants can run on even lesser RAM, I think.
6) You get 5 GB of cloud storage for free through its cloud service (UbuntuOne) once you sign up.
7) Installing new software is quite easy. You have to open an application called Ubuntu Software Center (USC), select any software you like, and USC takes care of the rest, provided you have an internet connection. And you get all sorts of software in USC.
8) Ubuntu has a really strong and helpful community which is sure to help out with absolutely any problem one can encounter while using Ubuntu. Ask Ubuntu is a particularly useful site, somewhat similar to Quora.
9) Don't know about Windows 8, but in Ubuntu you do not need to install additional drivers (for the most part) for your hardware. This works even if you install Ubuntu all by yourself and has got nothing to do with the OEM.