weird issue with dual boot windows 7 and vista

Hi.

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 up

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 type

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