Just a shot in the dark but you didnt by accident remove or resrtrict thhe Administrator Rights in the folder its trying to write to did you???

As far as i know i haven't done that, however if you give me some examples of how that would have been done, or where i can check that status; i'll give it a try.

Ok,
Before we go there let me understand the issue,

You have several computers linked together via a "HUB" or a "Switch" but not a Broadband Router.

On one of these computers you have enabled ICS to allolw all of the computers to connect thru this one.

The ICS computer is using "Dial UP" to access the internet. (if you have a broadband connection $25.00 sloves the issue and you can disable ICS)

Also ICS has some issues when DNS or DHCP is enabled on the same unit. check to make sure those services are not running