I had recently a significant problem while trying to upgrade a Windows 98 installation to Windows 2000 pro. Everything turn ugly (I clicked to fast on a button and could not provide drivers for my motherboard, the installation went bad in the middle of the update of system parameters), leaving the system in a very bad situation: Impossible to finish the installation, impossible to come back to the previous OS (it was already partly replaced) and the backup I had was so partial that I was not sure I could be confident.
That is when I decided to apply a solution that I knew existed but I did not try before. Knoppix is a GNU/Linux distribution that does not need to be installed to start on a computer. Actually, what they call a LiveCD is a CD-ROM that holds a full OS ready to start from the CD-ROM without any installation. Exactly what I needed!
A simple boot on the CD drive allowed to access to my hard drives, do the adequate backup, check the exact status of the installation and be confident enough to start a full Windows 2000 installation from scratch knowing that everything was fine.
Knoppix is recommended as a kind of insurance: Get yourself a copy on a CD-ROM. You never know when it will come handy.
Interestingly, even in the default distribution that I advise to use as a system toolbox, there is enough software to just play with multimedia files (I tried successfully to play MP3 audio and DivX video files). And there are also very useful applications like the OpenOffice desktop suite, Internet web browsers, a few games, a GDB debugger for those using the GNU development suites, etc.