Your best bet is to get an external drive.... they're pretty cheap nowadays:
http://www.newegg.com/Store/SubCategory ... ard-Drives (you still get what you pay for, stick with Western Digital, Seagate type brands). It's a pretty foolproof & easy way of having backup.
Or you can network your pc's together using a router. But if you're having issues with one PC, it may not network properly. Decent routers cost from $50 and up. If your PC's are not set up with a wireless card/modem, then don't buy a wireless router. A wired router requires a cat5e cable.
You can connect PC's directly to one another with a patch cable and software (most PC makers offer kits that help you move your files and settings to a new PC, its the same concept).
I guess your other options are to use USB flash drives (depends on the amount of data) or burning to DVD - again, a DVD can only hold a certain amount of data and re-writeables don't always work in different machines.