I'm confused when you mention Excel. What would Excel have anything to do with moving data from one SQL database to another?
The short answer is, if you need a C# service, then write a C# service that copies the data directly from one database to the other. The problem that you are trying to solve is not very clear.
Having said all that, and with my limited understanding of the problem, it sounds like what you need is a SQL job that is scheduled to run once a day that copies the data from one server to the other. Since it sounds like they are on separate instances, you'll just need to set up a linked server on either the source or destination database and either push or pull the data into the correct table(s).
EDIT:
Ok, so if a windows service is a requirement, that is perfectly acceptable. But, like I mentioned, you should forget about Excel. You wouldn't want to go from SQL->Excel->SQL if you have no other reason for the data to exist in Excel.
Here is some information on creating a windows service:
Easiest language for creating a Windows service
Here is a simple tutorial on accessing SQL in C#: http://www.codeproject.com/Articles/4416/Beginners-guide-to-accessing-SQL-Server-through-C
If you want a more formal solution (read: data access layer), I'd point you toward Entity Framework. The complexity of the project will probably be the driving factor on whether you just want to do SQL statements in your code vs. going with a full blown DAL.