Dual boot Vista modifying XP registry?

I have - and can replicate (I've managed it 3 times) - a problem whereby
Vista Media Extender installation/setup modifies a Windows XP registry
install key. The XP install is on a master drive and the Vista install is on
a slave drive. The key that is modified prevents the XP Welcome Screen and
fast user switching (the Ginadll entry in the XP Winlogon key). I've tried
once installing Vista from outside of XP with only the Vista hard driver
connected and I then used Acronis OS Selector. The problem didn't then
present itself.

I also notice that Windows Vista Media Centre automatically detects, by
default, relevant media files on the XP master driver without any prompting
i.e. it doesn't need to do a full search for random files on the XP install
master drive.

Also, having used the Vista dual boot screen to select the XP install
("older version of Windows"), installing Net Framework 3 on the XP install
results in files being written (without any prompting) to the Vista slave
drive. Subsequent deletion of those files results in a XP startup error
"could not load file or assembly
'MOM.Implementation" when using ATI Catalyst. If the files are left where
they are (on the Vista slave drive) the "could not load file..." problem
doesn't appear. But this really shouldn't happen, should it?

Could someone confirm that Vista installation/setup including
installation/setup of Media Extender can make changes to the XP registry on
a dual boot system? If this is the case - and my experience suggests that it
is or that something else odd is going - then why does it happen?

Gareth.