I am going to assume by your question that you are using Windows, the Write To Spreadsheet File.vi and MS Excel.
The Write To Spreadsheet File.vi writes data to a delimited (tab delimeters in my case) text file that is a format that can be read by most spreadsheet programs, but it is NOT Excel file format. If you attempt to open the file from Excel you have to confirm Excel's interpretation of how it thinks it should parse the data in the text file. It is usually correct, but you still have to go through the steps.
I have found that if I name the TEXT spreadsheet file with an .xls extension and open by association (double click on the filename in the Windows explorer. Not by file>open in Excel) then the text file will be opened without the parsing dialogs. Yo
u can exit the spreadsheet without any dialogs if you don't make any changes to it, but you will be prompted to save the text file in excel or text format upon exiting if you do make changes.
I use several Windows and excel versions. I have just tested this on NT 4.0 and Excel 97 which is in front of me and it works as described. Others?
That is a quick and dirty way of accomplishing what I think your asking. A more involved method would be to actually save the data in an excel format in the file. This can be accomplished by using ActiveX with Excel. There is much info on that in these Developer Exchange resources by searching on Excel and ActiveX.
Good Luck!