I moved from Vista to Windows 7 using EasyFileTransfer (which worked far better than I expected!!), changed computers, and this problem, which I saw for months on Vista on the previous PC has followed to the new one: If I double-click on any Excel (and only Excel, other Office apps A-OK) file, Excel starts, but then I get a pop-up message saying "There was a problem sending the command to the program". If I then drag the Excel file icon from (e.g.) the desktop to the empty/blank Excel space, it starts fine. If I open any Recent Document from within Excel, no problem. But clicking on any Excel file from a folder, or as an email attachment, no go. The error message pops up and I must drag it into the Excel window for it to open.
I'm thinking it's a registry error, but so far all sweeps haven't detected any registry problems.