External storage accessed by two PCs...HDD? NAS?

Hi, I have two PCs that need to have access to the same files. They are not networked together (I tried this once using the tool that comes with Windows XP Home...miserable failure!). In my ideal world, there would be some kind of external hard drive that we could both save to and access files from...like a server, but without setting up the whole network shebang. Does this exist? Is this a NAS, or would I have to figure out our network first? What kinds of products should I be looking at?

Both of these PCs are plugged into the same router to get to the internet, so if there's something I can plug into the router that we can both access, that would be cool too.

Many thanks for any suggestions or guidance!