There may be other ways to do this, but a small Linux, like DSL is the most available, and the easiest. An operating system on a pendrive gives you a super data-secure method of building an inexpensive, small computer for internet use and basic office tasks. It's secure because you can take, not only your newly created data, but your entire operating system away with you separately from your machine. It's also a great hobbiest's OS because it's flexible and free. Here's what I did with it recently.
Also, DSL and the mini-itx form factor just seem to go together like apple pie and vanilla ice cream. Take a look at this article.