I Have an Excel 2003 file with a line similar to this:

You don't need OAuth or the spreadsheet API. Google Spreadsheet allows data entry with a simple form, which means also that a HTTP POST will do the trick. You just need to prepare your spreadsheet to accept data entries via a form as follows:
You can test the entry now with curl if you have it on your system (replace the formkey placeholder with the formkey from your table):
curl.exe -v -k "http://spreadsheets.google.com/formResponse?formkey=" -d "entry.0.single=test&entry.1.single=test2&pageNumber=0&backupCache=&submit=Submit"
Next we try to execute the form POST from our Excel sheet via the following code. Add a reference to "Microsoft XML, v3.0" before. Replace column1 with your desired values.
Dim httpRequest as XMLHTTP
Set httpRequest = New XMLHTTP
httpRequest.Open "POST", "http://spreadsheets.google.com/formResponse?formkey=&ifq", False
httpRequest.setRequestHeader "Content-Type", "application/x-www-form-urlencoded"
httpRequest.Send "entry.0.single=" + column1 + "&entry.1.single=" + column2 + "&pageNumber=0&backupCache&submit=Submit"
'Check result in the following vars
httpRequest.status
httpRequest.statusText
Hope that helps