Vista - Vista SP1 & Mapped drive issue

We have a desktop (Ultimate) and a laptop (Business). The desktop has a
shared
folder that is accessed from the laptop. At seemingly random times the
laptop
suddenly cannot access the share on the desktop machine and so cannot save
files back to their original locations. Whenever this happens the desktop
needs
to be rebooted to make it work again. Also nearly every time the desktop
will
never shut down and you have to power down manually.

Is anyone else having this problem? If so do you know of a fix.

Thanks!!!

Keith.