Ok, after trying a lot of free librery i found a solution.
Excellibrary works! link to google code
I have tried it with Visual Studio 2008 in combination with a Windows CE 5 device and it created an xls file easily. I must test it to check the performance but at least it works and it's free. Also the sintax is very simple
Dim file As String = "\HardDisk\newdoc.xls"
Dim workbook As New Workbook()
Dim worksheet As New SpreadSheet.Worksheet("First Sheet")
worksheet.Cells(3, 3) = New Cell(3.45)
workbook.Worksheets.Add(worksheet)
workbook.Save(file)