Internet Connection Firewall and ICS service started, but not functioning

Hi

I have a Windows XP computer connected to the internet, and another
Unix machine accessing the Internet via the XP computer.
I have Internet Connection Sharing and Windows firewall enabled on the
computer running XP . Everything was working fine till a month ago
when I was using it.
Now however, I cannot access the internet from my unix computer unless
I stop and restart the ICF/ICS service through Services/Control Panel.
The service is set to startup automatically, and the status does show
it as "started" when I boot the computer. It doesnt write any errors
in the Event Viewer log.
Stopping and restarting the service does not change any of the routing
information as per "route print".

Any ideas on what might be going on?

Thanks for your answers.

Roy