It turns out that wikipedia makes all of its text not just available, but easy to download. Getting a browser for that text is a little harder, but there are some nice out-of-the-box wiki browsing apps for Windows.
Here are the parts you'd need to download and browse the full text (no pictures, unfortunately) of English wikipedia.
1. Download the compressed 5.7G wikipedia archive here: http://download.wikimedia.org/
2. Get the wikiTaxi application here: http://www.wikitaxi.org/
I haven't actually tried this since wikiTaxi is a windows app and I'm running linux, but I've seen several favorable reviews.
Alternatively, you could try Pocket Wikipedia, a much smaller text-and-pictures version that contains many of the most popular wikipedia pages.: http://www.free-soft.ro/
More info on downloading from wikipedia is available here: http://en.wikipedia.org/wiki/