Create a freestyle job. As only build step, have a Windows batch containing "set". Run and inspect the console output, look for the PATH environment variable.
Jenkins wrongly uses quotes when adding the current JRE path, e.g. in my case the PATH is shown like this
Path=C:\Program Files\Java\jre6\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Program Files (x86)\Git\cmd;"C:\Program Files\Java\jre6\bin"
Every command which tries to modify the path during a Jenkins job will fail because of those wrong quotes, e.g. you can no longer run batch commands like
setx PATH "%PATH%;c:\some\new\path" /m
because the quotes are incorrectly nested.
Running "set" outside Jenkins produces the same path without the quoted JRE path.
Jenkins version 1.527