First, try disabling the port forward, then try going to the IP address. It could very well be that the router has port 80 used for its admin interface and it is also serving it up to the public (bad idea). If you still get the router login after removing the port forward, you'll have to find the setting to disable external visibility.
Do your other port forwards work? Are you able to login via SSH to your desktop? If not, it may be the router not properly forwarding any ports at all. I had a router once that was simply refusing to forward anything. After failing to find any setting that may have been accidentally enabled or disabled, I ended up doing a factory reset, and that fixed the problem.