Not really clear here what you are asking is this an FTP program or DL that is interrupted ? If so there should be options in those programs for auto reconnect. In general if you are looking for a program that maintains connectivity on a connection that is more prone to dropouts like dialup I would suggest checking out Connection Keeper its freeware Connection Keeper - Keeps your connection alive and automatically closes popup windows.
why do you want to auto restart your computer when the internet is down? wouldn't you want to just restart the connection? Mac Daddy's suggestion could help. Either that or you can set the connection properties in Windows to just reconnect if the line is dropped. If you are using a router/gateway, it should reconnect automatically too.
