How to open workbooks in same instance of Excel 2007

Had to rebuild my PC's hard disk and reinstall software. Now, whenever I open another workbook via explorer double click, a new instance of Excel starts. If I open another workbook using Excel's menu open, then the workbook opens in the same instance. I've tried a 'Repair' of Office and resetting the default programs to Excel for all Excel related file extension types. Please help. Thanks.

Good try but that didn't work either. Thanks.

I have a very similar issue now that I had windows XP upgraded to windows 7. The checking of ignore DDE works for the day. however when I reboot I can't open any excel files by clicking them. I can reset the option and everything works fine again except the new file always opens in the original instance of excel.

Thanks, shg, but I am sure. I've closed all instances of Excel, started Excel, and made sure the box was unchecked. No change. I changed all of the default programs that have extensions related to Excel to point to the Excel.EXE file in the Office12 folder, but still no luck.