It depends what you mean by a backup of all files on your PC.
Do you mean backup the whole system, including Windows files and everything else.
Or do you mean backup just your personal files that you have created (word documents, photographs, music files etc).
If you mean backup the whole system, and you are on Windows XP, then this can be about 2Gb of files so you need to store it on a second hard drive or a DVD.
I use a product called Norton Ghost to take an "image" of my whole Windows XP disk. I save this on a second hard drive on my computer.
If I need to reinstall Windows XP I can do this in just a few moments.
BUT when I re-install from this image it overwrites whatever is there, so I need to save my pesonal files first before doing this re-install.
There are other products similar to Ghost, you can read more about Ghost here:
http://www.symantec.com/home_homeoffice/produc ts/overview.jsp?pcid=br&pvid=ghost10