Hold the hard drive up to the light and hand copy all the 1's and 0's in the exact order... If you want 2 copies, use carbon paper...
The ideas above are sound. You can also buy an inexpensive program, like
www.secondcopy.com (I like this for simple backups) and a large external HD as Hugh mentioned and set up an automated backup schedule. SC is a glorified copy and paste program, meaning it does not convert your files to a tape backup like stream that is unreadable. SC can do normal/incremental/differential type backups.
Off site backups are good too, just need to be aware that they do cost money, although the prices are not bad today.
Even if you don't settle on a backup plan today, it would still be wise to pick up an external drive or large thumb drive and manually copy your most important data for now.
BTW, it's not really the OS you need to worry about, it is the hard drive that will fail to the point of expensive recovery.