I just installed BD Free, version 1.0.21.1099, on Win7 Home Premium 64 SP1. I, too, get the javascript error when attempting to register. I read an earlier closed discussion. It suggested enabling js in IE. I just tried that in IE 11 as best I could find a similar setting: Tools | Internet Options | "Security" | "Internet" | [Custom Level] | "Scripting" | "Scripting of Java Applets" (*) Enable (default had been Disable). No luck. That earlier thread had a question from a staffer, asking if <https://my.bitdefender.com/en_us/free/> showed correctly. It does not: just 2 text entry windows and the ubiquitous FB and G+ icons, but no text. What settings in IE 11 need to be changed to make it show correctly, js to work, and presumably, permit the registration to function? Someone in that discussion fixed it using the Command Prompt; but, that should not be necessary for a program advertised as easily usable by the non-tech types.
(I am also a regular Firefox user, who only uses IE when I have to.)