On Sunday I was forced to reinstall Vista, owing to a problem with the Font folder. I tried to fix it (in a way only I know how – I refer to it as "the big hammer") and I broke it. Anyway, the "Upgrade" was fairly smooth, despite the 80MB Windows Update download I didn't want.
Today I tried to open a .CHM file, only to find the table of contents working, but nothing showing in the main pane. After a little help from Google, it turns out that the file will not display properly if there's a "#" in the name.
This is the second time I've run into issues with long file names and Microsoft. SharePoint does not allow the upload of files with a # in the name, and now CHM files won't open in Vista.
Something to keep in mind if you're battling with this.