It depends on how much data you want to back up, but I think the simplest option is an external hard drive. Something like this click here should do the trick for a reasonable price. You just plug it in to a USB port and Windows will see it when you open up My Computer. It probably comes with back-up software, or you can get Windows to do a scheduled back-up, or - simplest, I think - just do it manually. Should your old PC fail or you want to transfer the data to another machine, just unplug it from your PC and into the other one and again, Windows will see it.