If those "some words" happen to be German nouns, it looks like the program thinking you're typing German. German nouns are written with a capital, and most Norwegian words wouldn't pass the German spell-check.
Compare the language settings on "some PC's" and "other PC's".
On some PC's, using IE, when I write emails on Yahoo, the first letter of some words in the middle of a sentence gets capitalized (automatically and erroneously). Also, most of the words I type get underlined with a wavy red line. I tried disabling spell-check in IE, but that didn't help. I am located in Norway, but type in several languages.