I downloaded Audacity yesterday and when I opened it today a dialogue box opened up asking me to register with serial number, etc. To the best of my knowledge I haven’t received any information that would help me with this process.
Since I downloaded yesterday I assume I have the latest version and I assumed I installed from the .exe installer.
Audacity is free open source software. It does not ask you to register.
I suggest that you uninstall whatever it is that you have installed and run a full system virus scan.
Only download programs from trusted sources and watch out for sneaky adverts.
The official Audacity download page is here: http://audacityteam.org/download/