Microsoft Excel Populated by VB.net

I have an application that has for about 2 years been using a MS. Excel spreadsheet as its reporting tool. It does the sql query and then it posts the results into an Excel Spreadsheet. It does do some formatting and to date it has been running fine. ABout 2 days ago the two ladies who use it began to notice that it was slowing down and now a 30 second report is taking2-3 minutes. I have defragged and done a disk cleanup. This does not seem to help. I have also done a spyware check just on the outside chance that this could be an issue. If anyone has any ideas please let me know.  Thanks in advance.