Loss of control of Explorer/Task Manager

[Andrew]

3 previously perfectly functioning installations of VNC 4 on WinXP Home 
machines and one on Win2K have now ceased working.

The symptom is all of these:
- Start Menu button does not respond
- Task bar icons do not respond
- System tray icons do not respond, even hover over
- CTRL-ALT-DEL invokes Task Manager but first click sends it out of focus 
and it no longer responds

I have tried two separate VNC Viewers.

A common factor could be that a well-meaning IT guy has been around these 
machines and has probably applied latest Microsoft patches.  Does anyone 
know if some "patches" have recently been released that screws with VNC?