FIXED THIS ISSUE ON MY MACHINE.

Only after -hours- of tinkering.

I was getting the "An erroc occurred when enabling internet connection
sharing. (null)" message also, when attempting to enable ICS on a Sprint
Mobile Broadband connection, to share with the XBOX 360 over ethernet.

I have Windows Live OneCare installed for firewall, virus, spyware
purposes... so, I had Windows Firewall disabled through services, since I am
using OneCare for firewall.

I enabled Windows Firewall, and stopped getting the error message. ICS is
working fine.

Sooo... a step to try, if you have Windows Firewall disabled through
services, enable it.