Getting "Working Offline" message

I just started getting a pop-up telling me I'm working off-line and giving me the option to login to my back-up dial-up account. I have DSL. I can close this message box and uncheck the "working offline" option under "File" in my IE browser and everything resumes as if nothing had happened, but it's annoying.

I'm using a router which I just added a new laptop to yesterday, which was coincidentally (?) about three hours before I began experiencing this problem. I get the pop-up (on my old desktop, not the new laptop) whether or not the new laptop is turned on.

Any ideas or simple fixes?