I'm trying to find out which of our PCs have internet access.
Normally, our ISP has ACL on the routers (we can't access those ourselves) to allow or deny internet access based on IP. Last week we discovered those rules didn't work anymore and everyone had access. After our complaints it works again, but we want to monitor the access to be sure that when this happens again we immediately (the same day) know about it.

I don't know about any usable values, system variable or regkey that can be useful, so I'm thinking to either use netstat on a remote PC and filter out any destination IP outside our network, or use psexec to make the remote PC send a ICMP request to an internet address and catch the reply.

In case of netstat, we only see if the PC has an internet connection at this moment. That's actually not really the right information.
In case of the PSexec, I can't get the reply into a variable. It works interactively, but since there are 483 machines to test, that's not the way to do it.

I'm using Windows7, but all are PCs use Windows XP with no PowerShell installed.

Any suggestions?