Sometimes Windows is just too expensive when building a custom pc, It will set you back $100. How about a free, open source linux distro.
Ubuntu is by far the most popular operating system. It is very lightweight and simple.
Puppy Linux is a small linux distro that can fit on a 256mb usb stick and works well on older components.
Mint Linux is a good, colourful linux distro.
Fedora is a nice lightweight distro.
Many, many more
There a thousands out there. Find yours!
For your inner geek