The error indicates that Inkscape is not using the included python
bundle to run the effects. We ship python with Inkscape in order to
avoid issues like this. Where did you download the zip file? Is there a
folder called "python" in the root inkscape directory?

In 0.45pre2 the effects didn't work either, and then I installed Python24.
I uninistalled Python24 from the Add/remove programs window, and then unzipped the new 0.45-1 version to the C;\Program Files\Inkcape path (it's just unzipping, doesn't really ' install' anything)
It does indeed give me a folder called python.
If I run the groups to pngs script I get an error message which refers to the non-existing C:\Python24. Also, Inkscape then freezes

Traceback (most recent call last):
  File "share\extensions\g2pngs.py", line 81, in ?
    e.affect()
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 153, in affect
    self.parse()
  File "C:\Program Files\Inkscape\share\extensions\inkex.py", line 114, in parse
    self.document = reader.fromStream(stream)
  File "C:\Python24\Lib\site-packages\_xmlplus\dom\ext\reader\Sax2.py", line 372, in fromStream
  File "C:\Python24\Lib\site-packages\_xmlplus\sax\expatreader.py", line 109, in parse
  File "C:\Python24\Lib\site-packages\_xmlplus\sax\xmlreader.py", line 121, in parse
IOError: [Errno 9] Bad file descriptor