4 out of 12 computers will not now run VNCViewer

Hi, this is an out of the blue problem, the last few months no problems, overnight several computers (4) on my net get the "Error creating local image of screen" message.
I have updated to 2009.7.24 build and installed the latest VNCVewer.exe to no avail.
8 other machines are fine. I have reinstalled VNC on the 4, they ping OK and I can run the VNC Java Viewer on them. There have been some MS udates applied but this covers the other computers too. Of the 4, 2 are XP and 2 Are Vista.

Any thoughts appreciated.
Mike