Can anyone tell me what this program with the weird name does?

Not sure why you are asking here. But a search of Google would suggest it is an install tool for registering DLL files. Seems it self deletes upon 2 reboots.

Thanks for the quick response. I bumped into the program in the MP3tag program folder.

Quote from Wikipedia:

Nullsoft Scriptable Install System (NSIS) is a script-driven installer authoring tool for Microsoft Windows backed by Nullsoft, the creators of Winamp.

I'm using NSIS as system to build the installer for Mp3tag and the RegTool is used in some cases for the installation and uninstallation of the Explorer shell extension.

As @MotleyG pointed out above, it self deletes upon 2 reboots when it's not longer needed to do its work.