ICS service has vanished from WinXP

We've had ICS working for quite a while, then suddenly the slave PCs
couldn't dial via the master (all running XP SP1).

When I re-did the ICS wizard (using an Administrator account), it came up
with the following message:-

"Cannot enable shared access.
Error 1060: The specified service does not exist as an instlled service"

When I looked in Services, it wasn't even listed. I tried to add Windows
Components in Add/Remove Programs, but couldn't find it in there. I'm
wondering -

1) How do I re-install it
2) How did it disappear?

Any clues on either would be gratefully appreciated.

Many thanks, Jan