Well, you can start by discussing both operating systems' history, mentioning the start of Linux as a hobbyist project back in the 1990s, which built upon twenty years of Unix development, while Windows 7 draws upon the Windows NT line of operating systems, and is largely disconnected with the operating systems which came before Windows 2000. Then, there's the two different development approaches: Windows 7 having a closed-source, in-house development programme which makes the system very consistent throughout its user interface, but leaves little scope for customisation; whereas Linux uses an open-source model and draws upon many different sources, from the GNU project and the Linux kernel program, to the GNOME, KDE, XFCE and other desktop environment development programmes, to software from such disparate sources as Oracle and the fruits of labour of bedroom coders.
Then you can go onto the different structures which both OSes take to their user interface. While Windows 7 integrates its desktop environment deeply into the rest of the system, Linux builds it as a layer on top of the command line interface, and while such distributions as Ubuntu have obvious preferred desktop environments (Unity in the case of Ubuntu, and formerly GNOME), it is possible to pick one of many desktop environments in most Linux distributions. Indeed, it is possible to run a Linux system on the terminal alone, and even easily control it over the internet using such tools as SSH. This makes Linux an ideal choice as an operating system for a server, as it does not require the additional overhead of a graphical user interface to run, and unless the system's completely borked, one can fix it without having to be physically present in the room with the system
This gives you the room to segue into a discussion of the differences between the approaches to user interface. While the in-house development plan of Windows 7 allows the user interface to be fairly consistent throughout, the different desktop environments of Linux have different approaches to their UIs. Unity is very different to GNOME, which is different to KDE, which is different to something lightweight like XFCE or Fluxbox, which is different again to something like Étoilé or FVWM. While the more heavily developed desktop environments do tend to be consistent within themselves, the ability to pick and choose between software packages does sometimes give a tendency for software to be inconsistent in UI approaches.
This is just a start, and there are plenty of other things to discuss, such as the /dev/sdX vs. A:, C:, etc. approach to filesystems, the use of Ext3/Ext4 vs. NTFS, the DirectX integrated approach to multimedia vs. the OpenGL, ALSA, etc. approach of separate packages for each bit of hardware, the readily available patented multimedia formats in Windows 7 vs. the slightly surrepitous approach you have to take in Linux, and so on and so forth. Hope this helps somewhat.
