You might have heard about various version of Linux going portable in Pen Drives, CD Rom or DVD drives, and all this booting live on your system and giving a full experience of Linux even without installing it. But have you heard of Linux running live in your computer, along with Windows. Just like Hackintosh works to run Mac inside Windows powered computer.
There is a release of Ubuntu by Claudio César Sánchez Tejeda which can be used inside windows just like a virtual OS inside a given OS. This release is of about 440MB in size and takes upto 1GB on extracting. You can download this Free Operating System from Sourceforge.net. For documentation and relase notes visit here.