Chris I'm also having similar problems on build 5536 of Vista Ultimate:

1. No caching. Performance miserable. Especially with 200gig+ of mpeg4-asp
avi's that it wants to thumbnail and generally bring my sys to a grinding
death.

2. Every time I want to optimise the 360 using the optimise within settings,
the 360 gets disconnected and nothing appears to happen.

3. Really, WMV and mpeg1/2 support is so narrow. Where is the mpeg4-asp and
avc support.....Divx was problematic, XVID compile v1.2 with the smp patch
seems to work ok. CoreAVC v1.5 is ok but will crash WMP11 in some cases of
shutting down before the footage is finished.

4. How the hell does a user disable the performance robbing thumbnaills
within media centre? I'd prefer not to slow down everything and just go by
filenames.