SIS on Windows Server 2003 R2 (not WSS)

Have you considered doing some port forwarding on the firewall? Instead of opening another port you could redirect 443 for that IP on the firewall to something else on the machine (whatever port SequeLink actually wants to use) and just not fight with it?

Enabling the "Allow users to connect remotely using Terminal Services" policy setting will cause Windows XP machines to which the policy applies to begin allowing terminal services logons (assuming that you're not blocking inbound TCP port 3389 with a Windows Firewall or other firewall setting).

The "Local Policy Does Not Permit You to Log On Interactively" error you're receiving is best mitigated by nesting a group that contains the users who are allowed to logon via Terminal Services into each Windows XP computer's "Remote Desktop Users" group.

You can do this group nesting with Group Policy. Use the "Restricted Groups" functionality (under "Computer Configuration", "Windows Settings", and "Security Settings") to perform the nesting. Make an entry for the "DOMAIN\Domain Users" (or whatever group contains all the users who should have Terminal Services logon access) group there. In the properties for that entry add "Remote Desktop Users" in the "This group is a member of:" section of the policy.

I'd recommend, initially, applying this policy only to some test computers. You can verify it's working by examining the membership of the "Remote Desktop Users" group on the affected computers after the policy has been applied. Once it's to your liking apply it more broadly.