I have a very similar setting but I use DBD::ODBC instead. Do you define an ODBC source in the control panel of your web server box? This is my connect line:

$dbh = DBI->connect('DBI:ODBC:odbcSourceName', 'username', 'password')
+ 
  || die $dbh->errstr;
[download]

I would recommend that you switch to DBI. Installing DBI and DBD::ODBC is a snap from ppm. Using DBI instead of Win32::ODBC has the advantage of being more portable.