Vista - MP4 files: Import & Play in WMP11 & VMC ??

I have installed the Haali splitter & ffdshow support and made registry
entries to allow MP4 video files to play in WMP11 & Vista Media Center (VMC).
This works OK. However I have two annoying problems:

1. VMC & WMP11 both extract video previews that are mostly just black screen
images, I presume because the first frame happens to be a black screen. This
is not very useful - can I force the extraction of an preview thumbail from
further into the file (say Frame 100)?

2. On one PC these MP4 files are being imported by WMP11 into its library
under the "Video" section which is what I want. However on a second PC
WMP11 is importing the same files into the "Other Media" section of the
library. This is very frustrating because I can't see why the two PC's are
different. What section in the Registry defines how & where media files are
imported into the WMP11 library?

Thanks
NigelS