Installing and upgrading help

I have same problem. In my case external db is MSSQL Server and i have tested the setting in moodle external db view thats perfectly connected. I tested my auth with test db and test table having less user records and it worked well. BUT when i connected with actual table having large amount of users and run sync_users.php file  its takes lot of time first (as i set appropriate max execution time settings in php.ini file) then it gives this error Warning: mssql_connect(): Unable to connect to server /lib/adodb/drivers/adodb-mssql.inc.php on line 616. My moodle intance and external db both are on windows OS.

Have you resolved this problem? Or any one can please tell me about the resolution of this issue.