vncviewer connects but can't log in desktop

[Barry Yu]

In the office there are 6 machines, vnc viewer can remotely connect to 
all of them without problem except one XP pro, the problem is
when connected to this machine after had entered the password the 
process began to logon, then encountered a full black screen, with
a very very tiny square taking place of mouse arrow, this mouse pointer 
can move as usual but just can't enable me to log on the desktop
of machine.
I compared this machine with another one that also use XP, found that 
this one is with the screensaver activated, when I disable the
screensaver,  I can log on the desktop now.
My question is : If that XP must have the screensaver activated ( That 
machine's user preference ), is there a way for vnc viewer to log on?
or this problem is not necessary the screensaver of XP but something else?
tks.