Windows: Backing up just your data is better than no backup at all, but creating a complete disk image allows you to restore not just your data but your entire operating system and all your settings. SelfImage can help.
SelfImage is an open-source tool for creating disk images. Not only can you create a simple disk image of an inactive partition but you can also create images of partitions in active use and even partitions not recognised by Windows. This allows you to use SelfImage to not only backup your Windows partition from Windows but also to create images of partitions that Windows doesn't normally recognise like a Linux and Mac OS X partition on a multi-boot system.
SelfImage is open-source, Windows only. Thanks Mike-RaWare!