weird issue with dual boot windows 7 and vista



On a number of identical computers, I've attempted to create a dual
boot system, where
I've used an acronis backup of vista on a separate partition in order
to use it in combination
with a windows 7 upgrade over the original vista version, to create a
dual boot system (using easybcd 2 and easeus partition master).
So for instance, the disk is 300 gb, I've divided it into 3
partitions, 100 gb each (shrinking the original vista partition) and
ensuring that the partition where I intend to restore vista later on
is slightly larger than the original C partition, where the vista
version will be modified with the windows 7 upgrade.
On 3 out of 6 computers, it works fine and I can use both without
problems, being able to update
both vista and windows 7 without problems.
On the other computers there is a weird problem with vista, while
windows 7 works fine.
When I try to boot into vista, everything seems to go ok, and I end
at the login screen.
Then when I log in, the screen remains black and only the mouse
pointer is visible.
I can get into the task manager with Ctrl-Shift-Esc, and I can do
things like start a command prompt.
But the screen remains black and often I get errors when I try to
in a command via the task manager menu option "run..".
Does anyone know how to resolve this issue and fix the vista system
somehow on the 3 systems where this issue seems to occur?
Thanks in advance for any suggestions, N

My Computer