Hi there!
Thanks for you help guys, @sazp96, @kennethlimcp! I figured it out with your help!
I didn't install 0.10 because it was stated nowhere. So was it for Git....
In other words; this guide is incomplete: http://docs.particle.io/core/cli/ (see below for the complete windows version 
)
For me, it remains strange that I've to install 6 different programs in order to change the WiFi password on my spark and use the serial monitor... (that's all I wanted 
).
Few notes:
- When installing a program - do not change the installation path.
- On win 8.1 - .NET 2.0 can't be installed (easily). luckily 4.5.2 does the trick for me!
- In the end installed node.js 0.10.
So to summarize, you have to install the flowing:
- Install node.js 0.10 (just to be sure). It's a little bit hidden so here's a link: Index of /dist/v0.10.38/x64/ (download .msi)
- Install OpenSSL: https://dl.dropboxusercontent.com/u/36134145/Spark-cli/Win32OpenSSL-1_0_1h.exe776 (source)
- Install Visual Studio Express 2013 for Windows Desktop: https://dl.dropboxusercontent.com/u/36134145/Spark-cli/vs_community.exe34 (source)
- Install Git: Git - Downloading Package
- Install .NET 4.5: https://www.microsoft.com/nl-nl/download/details.aspx?id=42642 (if you'r on [< win7] and it doesn't work try .NET 2.0)
- Install python 2.7.9: Python Release Python 2.7.9 | Python.org
- Open ' node.js command prompt' and type
npm isntall -g particle-cli - That should do the trick!
(I'm sure spark... eh.. Particle can do better than this! 
)