I see some apps drop windows xp support, and I wonder what exactly it means. I'm still trying to learn winapi from 2004, and I wonder what changed since then.

Run those apps on XP (VM maybe) and see which functions are missing? You'll have to patch each function after finding it to find the next one though. (just change its string name for testing purposes to something valid, easy search/replace in Hex Editor)

Also, Vista made it possible to use "direct" low level TLS (without APIs) in DLLs loaded with LoadLibrary. Previously this worked only on executables or DLLs loaded statically.

Don't have enough hdd space for VM. Hoped there is a list somewhere.

TLS is very interesting, thanks!

If you have some old versions of the MSDN that will list all the APIs supported at the time, and which previous versions of Windows each API is valid for.

Search and install the "msdn library for visual studio 2008", I believe it is where you can find the last available offline version of the win32 API documentation.

Once you install this (well, if you are willing, that's 2GB), go to the internal search engine and search for "functions by release".

You will find a list of functions introduced in each version pre-windows 7.