My immediate thought is whether researchers or investigative journalists will find cold hard US government backdoors. This is potentially big.

They won't. Such backdoors would have to be hidden from Joe Average Coder at Microsoft first and foremost. Coding for MS is not a livelong thing, you know? So any backdoor would a) be obfuscated and b) have some form of plausible deniability. If I would have to make them, they would look like strings of two or three bugs.

Likely you're right, but let's see if anything can become plausibly demonstrable after obsessive scrutiny. Like many situations in life, it may depend on whether someone determined / resourceful enough wants to do this. There may be no one with sufficient motivation.

Also, it's not just (allegedly) all of XP source that's been leaked:

https://www.bleepingcomputer.com/news/microsoft/the-windows-...

It's also Windows Server 2003, MS DOS 3.30, MS DOS 6.0, Windows 2000, Windows CE 3, Windows CE 4, Windows CE 5, Windows Embedded 7, Windows Embedded CE, Windows NT 3.5, and Windows NT 4!

That's a huge amount of stuff to analyse.

Do you know if there is source code for classic Windows apps like the Calculator, Notepad or Paint? Would love to recreate those simple apps and chuck the lousy Linux/Mac equivalents.

As a recall, making notepad is like a homework assignment for a visual basic class. You just drag the text editor window and add the menus, there isn't a whole lot there!

Paint would probably be a bit more work, but there are a few clones out there already