In addition to what Andrew had suggested, you may need to do following

Right click on empty space in your DTS designer window

Choose "Disconnected Edit..."

Open up the connections

Open up your Excel Connection

Look in the OLE DB Properties of the connection

The tenth item in the list is Extended Properties

Now look across to the right hand pane and you will see a table with two columns. The one we want has value in the leftmost column and mine has "Excel 8.0;HDR=YES;"

Double click on the value and change the string to read "Excel 8.0;HDR=YES;IMEX=1"