I've had more time to think about this answer, and would now recommend using an XML approach with the Open XML Office spreadsheet format.
Heres some good links to get started on building a office document with code.
Just use SSIS on SQL Server. It provides the ability to export to Excel.
Don't run office on the server. Alteranatively waste money on aspose or spreadsheetgear.
GC does work your just not using it properly follow this pattern...
private void killExcel()
if(xlApp != null)
xlApp = null;
get your Excel operational class to implement IDisposable, and then stick killExcel() in the Dispose method.
UPDATE: Also note that sometimes dev will still see Excel.exe running in task manager. Before assuming the above code isn't working, check that the process that is running the code is also closed. In the case of a VSTO or COM addin, check that Word/powerpoint/other excel instance is also closed as there is still a GC root back to the launching process. Once that is closed the Excel.exe process will close.