If you don't have a spare portable drive, you can back up to disc (dvd,cd,floppy disk) - this is very important if you need to re-install operating systems to iron out any bugs. You should always backup anyway to restore own files otherwise you'll lose everything. Some programmes cannot be backed up so you will need original discs.
Portable drives are reasonably priced but dvd re-writables (which I use as well) are an extra backup - cds and dvds are most reliable so long as they don't get scratched. 8-)