By "registration", I'm referring to what happens when SONAR scans plugins. Some background...
Each plugin has an initialization routine built into it which Plugin Manager invokes during the scan. The DAW cannot know what any given plugin has to do to prepare itself for use, so the DLL itself has to provide that functionality. Plugin Manager can only sit and wait for the DLL to report back that it's done registering itself. If anything goes awry, Plugin Manager won't know why and can do nothing but de-list the plugin.
One thing that can go wrong under Windows 7 is that a plugin may need to write to the HKEY_LOCAL_MACHINE registry hive. Under previous versions of Windows, this was common practice and perfectly OK. But under Windows 7, it's a no-no. Consequently, older software can have issues with Windows 7 and may only install and/or run when the user is Administrator.