So I found the "owner" of the HTML Help component in Windows (I put owner in quotes because this tool is _very_ maintenance mode, so nothing short of a critical vulnerability is going to be touched) and he helped me do some debugging.

In short, the index file is either corrupt, or it is not parsed correctly. I can reliably reproduce the crash with the following steps:

1. Open the Python docs
2. Switch to the Index tab
3. Double-click on any second-level (indented) entry
4. Modify the text in the Index search box

We haven't yet gone deep enough to be able to tell whether the help compiler is at fault, or the loader.

If anyone has the time and inclination, testing with older versions (if you can find them) of the HTML Help compiler may find a version that works. Unfortunately, the older copies we used to use were on the Subversion server, which is long gone now.

Alternatively, if anyone knows of a similar tool that we can redistribute easily with CPython (i.e. it's not as big as Zeal) and is significantly better than just using the user's default browser, I'd be interested to hear about it (as would the rest of the developer world, I'm sure - this is a fairly popular format!).