I should know this, embarassing.
You need to make XMLHInit return the XHR object. I'd also choose better names for your functions and variables. Try not to mix upper and lower case so much and just make them simpler but still easy to identify what they are for.
I don't think there's anything wrong with making a POST request along with GET stuff in the query string.
Above all, you need to learn to debug this sort of thing yourself. Go through the entire procedure step by step, and use alert or console.log() to check that things are being defined, that variables hold the values they should be holding, and so on. You'll probably find that your JS console is showing errors anyway, due to Passw_XMLHttpRequest being undefined in the ondownloadrequest function. If you have not used Firebug before, I strongly recommend you use it and become familiar with it, at the very least with the console's ability to show you errors.
Thank you. I know about Firebug, for reasons I do not recall I couldn't install it the previous time I tried. This time it worked though. It reports the error you mentioned.
I have tried (with questionable attentiveness) to debug it with "alert" but didn't catch the issue I should have catched. And I'd say I have fairly good practice in debugging in general. The XMLHInit function was added as an afterthought because the code did not work in a previous revision, so it is likely there are more things to debug. I will definitely try the console.log() function with Firebug.
P.S. And certainly, I'l clean up the names.