Ubuntu is a solid general purpose operating system. It is the closest you can come to getting an operating system that is more stable than Windows or Mac, more customisable than Windows or Mac, and supportive of all the same standards as Windows or Mac.
If you go with Windows, you are supporting a company convicted of monopolising and will be paying heaps of cash for the operating system, virus software, malware protection, and any productivity applications you might want to use.
If you go with Mac, you pay more for the hardware, more for the operating system, and are still vulnerable to security issues. Apple suppress security issues rather than letting them into the open so they can be fixed (see George Ou).
The skinny is Windows and Mac are both preparatory, Linux, is open source. Ubuntu is based on the Linux kernel. It is freely available, free to customise and, thanks to the Linux Foundation, will remain so.
If you want to give it a go, you can download the LiveCD image at ubuntu.com or try Ubuntu Studio, the new multimedia flavour of Ubuntu. If you do not have broadband, you can ask for a free CD to be shipped to you (no charge for the shipping): shipit.ubuntu.com
