Hi,
I've over 10 years experience with building .net applications and especially web-enabled applications for Windows PCs and servers.
If I understand your requirement correctly, I think that a Windows service would be a good way to implement this. Windows services are easy to install (just an install wizard - click "next" two or three times and it'll install). Once installed, it runs behind the scenes and is invisible to the user, and will start automatically when a machine is rebooted.
Monitoring the IP address is straightforward and emailing users of an IP change is fine, once we have details of the email server to use. As I'm sure you're aware, most office networks (LANs) generally won't change IP address - the local machine will be allocated an IP address on the network, but the internet-facing IP address is usually allocated by the LAN. So I'm guessing you either want the LAN IP address monitored, or the internet-allocated one in the case of home users?
For the HTTP proxy, I'm guessing you want a local application on the PC to send HTTP requests to this service, which then proxies them on to CLAD (which will have a static endpoint somewhere) and returns the messages. This shouldn't be a problem, unless the HTTP connections are secure (HTTPS), in which case an alternative implementation may be required. It would be great if you could share a bit more info on the communication with CLAD so I could confirm these assumptions?
Regards,
Chris
PulseLab Ltd (UK).