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.

After some experimenting I ended up with the following
which works great. I had to add the ping line to add a delay so that
Excel 2003 has time to open  before the command to open the target file
is executed:

start "C:\Program Files\Microsoft Office\Office12" excel.exe
ping -n 5 127.0.0.1>nul
start /d "C:\Documents and Settings\Pete\My Documents\PD-
Trak\Development\6.2.2" Open_PD-Trak.xls

I also had to add an underscore in place of the blank in the file
name. I tried using %20 instead but without success. A minor
compromise. Thanks again!