Spaces In Pathnames
Notepad++ does support long file names with spaces. There shouldn't be a problem if you open a such file via the internal File -> Open command, via the Drag 'N' Drop or via the context menu (right click mouse, installed by Notepad++ Installer) under Explorer.
However, if you try to associate Notepad++ with some file extensions manually, there may be a problem. According to my experience, if you want to add a registry key by yourself, you should do something like that (with the double quotes) :
"C:\Program Files\Whatever\notepad++.exe" "%1"
to make sure the shell correctly interprets the string.
From v2.6, Notepad++ integrates the File Associations Manager which allows user associate/disassociate all the file extensions he/she wants with Notepad++. Note that you should have Administrator permissions to perform this operation.