First, I don't think you can do this if you already have Excel 2007 open.
Windows looks for the program name "Excel.exe" NOT versions.
But as to your question:
You've got Excel 2007 and Excel 2003 installed  in two separate directories.
You need to determine the LOCATION of each application (or at least 2003).
I created a bat file  called "start.bat"
In it, I put:
"C:\Program Files\Microsoft Office\OFFICE11\EXCEL.EXE" "C:\Documents and
Settings\(myusername)\My Documents\data\TestFile.xls"
this bat file will then open  the file using the Excel application from the
location specified.
Try something like that.