My previous OS was Vista and I couldn't connect my Canon 20D camera without going through Picasa 3. So when I upgraded to Windows7 I was pleased to see that Canon said no driver was required. However, they do not say that your communication settings must be changed before the OS will recognise your device.
So after plugging my camera in and discovering that Windows needed a driver, and then wasting ages searching for a non-existent driver, I discovered an obscure post by Jeff1212 that solved my problems immediately.
What Canon do not tell you is that before Windows7 will recognise your device, without a driver, you must change the camera's communication settings by going to Menu --> Communication and selecting PTP instead of Normal.
Once you have done this, plug your camera in and the OS will recognise the device and allow you to navigate through Computer --> Canon EOS 20D --> Removable Storage to your picture folders.
Picasa 3 is still a great free option for downloading your images, but the simple fix from Jeff1212 is great for quickly viewing your files.