Many ways of doing that-
1) Get a hard drive and make it a slave drive. Then you can copy your hard drive(folders) on to that one.
2) Use system backup utilities such as norton ghost, drive image etc.
3) Burn the files you want to backup on a cd-r/cd-rw
Personally, I dont think you need to back up anything (mandrake is really easy to install) although its a good idea in case something goes wrong.