Then start fresh on the one computer. A lesson that many have to learn first hand is finding out why one computer does this can take longer than a fresh install.
I have setup a simple VPN using windows xp as the "server". I have forwarded ports 1723 (TCP) and 500 (UDP) to the vpn server on our linksys router. I am able to connect to the computer and access the resources using the Windows XP VPN client but I get automatically disconnected after 1 minute. I tried changing the VPN "server" to a different XP box in the office and I can connect and stay connected without any issues. So, there is something with the one computer that is making the VPN disconnected after 1 minute.
I did a bunch of research on this and found that this was an issue after you installed SP1 and it was resolved in SP2. We are running XP with SP3 so that cannot be the issue.
I also found an article about changing the MTU size but that did not fix the issue either.
Has anyone seen this and more importantly found a way to fix it?