Responding to both your posts here. 1848 is the latest official release (AFAIK); 1862 and 1884 are public betas. You might want to keep an eye on the announcements thread (
http://forum.parallels.com/forum27.html) as well as the updates page (
http://www.parallels.com/en/download/desktop/update/). Build 1884 is what's currently posted there, and is where I picked up 1862, which unfortunately no longer seems to be available (unless there's a public archive I don't know about).
Re #2, if you've already created a disk image with build 1848 and installed Windows onto it, keep it around! That image is still usable with subsequent builds (at least so far). No need to re-install Windows! All the same, should you ever need to reinstall, there shouldn't be a problem with you re-entering the key if you have a valid license and you're reinstalling on the same "machine". Entering the product key is not the same as registration with MS, which I believe is a choice you can opt out of and I wouldn't do more than once anyway.
- Bill