I can't tell you if it's related to XP environment but what I can say is that your windows path doesn't look good:
You may use :
"C:/xxx/yyyy/zzz" or "/c/xxx/yyyy/zzz" but I don't think "C:\\xxx\yyy\zzz" is correct.
Try this and let us know.
Thanks Loic for your answer.
Anyway, it wasn't a path problem (even the Windows-style path works).
I discovered that vbs does launch jar files and js doesn't.
So I created a vbs script (let's say javaLauncher.vbs):
Dim oShell, oCurDir
Set oShell = CreateObject("Wscript.shell")
strPath = Wscript.ScriptFullName
Set oCurDir = CreateObject("Scripting.FileSystemObject")
Set objFile = oCurDir.GetFile(strPath)
strFolder = oCurDir.GetParentFolderName(objFile)
oShell.CurrentDirectory = strFolder
Set oShell = Nothing
oJavaApp = new File("c:\path\to\myfolder\javaLauncher.vbs");
...and that works!