Repair windows jnlp file association
If suddenly your jenkins slave jnlp file wants to start via adobe reader instead of a reasonable program, your association between file extension and program is broken. No way to repair it via gui, so do this within an administrative command shell under windows 7:
assoc .jnlp=jnlpfile ftype jnlpfile="C:\Program Files\Java\jdk1.8.0_172\jre\bin\javaws.exe" "%1" %*
After shat you can right click on the jnlp icon and run open with, the javaws should be the default now.