I found the basic idea here, the exact steps are:

1. Install Portable 7zip
2. Download Oracle JDK installer for Windows (*.exe)
3. Run 7-ZipPortable.exe from your Portable 7zip
4. In 7zip find and right-click the jdk installer exe file
5. From the context menu use 7-Zip – Open Archive and then Extract
6. Now extract the resulting “tools.zip” to a folder that is writable for you
7. Open a cmd.exe, cd into the folder and execute this:

for /R %f in (.\*.pack) do @"%cd%\bin\unpack200" -r -v -l "" "%f" "%~pf%~nf.jar"

Kudos to Nick Russler for figuring out this tricky unpack200 command line!