Did you try unplugging the modem and plugging it back in?
Try the repair operation in the Windows networking utility? (Not sure how this works in Vista, never used it thank God.)
Try opening a command prompt and pinging somewhere like
http://www.google.com? What were the results?
If that failed, did you try pinging 127.0.0.1? What were the results?
If pinging 127.0.0.1 failed, you need to do a winsock repair - again, no clue how that works under Vista.
Try calling your internet service provider to see if there is an issue locally that may be causing the problem?
Check to make sure that you set up the ethernet connection in your network settings? (It should just automatically configure itself that way, but hey, we ARE talking about Windows here.)
Try asking your sister if she tried to make things better under Internet settings?
Try cleaning out the tubes?
Try using the air conductivity spray that makes wireless internet work better?