As you know, some of the latest viruses like to go around scrambling your hard drive.
If you have a backup hard drive attached when you get a virus, it usually scrambles that too.
One of the best ways to have a backup is to have it on another computer in another place. That way, even if your computer gets eaten, it can't spread to the other computer.
CrashPlan does this for free. They have paid options as well of course so you can use their cloud; but you don't need it.
If you don't have willing friends or good broadband, you can also install it on two computers that you own - say a desktop and a laptop. Chances are that if one of them gets eaten, you'll find out about it before the virus can attack the other one.
The other advantage is that if you accidentally overwrite a file, or your spreadsheet or database magically corrupts itself; CrashPlan keeps multiple daily copies.
There are other programs out there that do similar things like Carbonite, DropBox, etc but with those you usually have to store your files in their cloud.
CrashPlan is at http://crashplan.com/
Two caveats. 1 - it is a bit RAM hungry, and 2 - if you have to pay a lot for data, it might not be feasible to use it over the Internet - you can restrict it to only work when both computers are on the same network.