Offline Installers: Invalid Digital Signatures

In this instance, it's an expansion of the use of SHA-256 within the signing pipeline that Vista doesn't appear to support.

The bottom line is that we're at an inflection point in the evolution of commercial cryptography support where we have to move our technology choices forward in order to be considered trustworthy by modern operating systems. 

This means that we will ultimately need to choose between legacy operating systems at or near the end of their support life, or we have to completely bifurcate support in order to sequester the insecure legacy environments, which become increasingly vulnerable to proven cryptographic attacks, as research moves beyond proving theory to building efficient, practical attacks.  Evolution in cryptography is nothing new, and this kind of change has played out many times over the years.  History provides an effective guide for how it will ultimately play out, and practical attacks against SHA-1 are within reach at this point.

We have not reached a decision at this point as to whether to undertake the expense and risk of bifurcating our distribution infrastructure to support a less secure, legacy pipeline, for the sole  sake of continuing to support WinXP (Vista adoption is miniscule in comparison, and a no-brainer taken on it's own).

Regardless, barriers to continuing to run trailing-edge operating systems on the modern web will be met with ever-increasing amounts of friction and risk, as you miss out on the benefits of 10-15 years of security research, which are frankly necessary in the context of the modern Internet.

If you're cost sensitive and/or cannot afford to upgrade to a modern operating system, modern Linux distributions like Mint Linux offer a free solution that includes modern operating system level security mitigations and cryptography support.  If you prefer Windows or Macintosh, a decent entry-level Win10 laptop can be had in the $400 range, a decent entry-level Chromebook will run you about $250, and if you'd prefer an inexpensive alternative to retail Apple prices, Walmart sells refurbished Apple products through their website at a wide range of ages and prices.