@Yunchen
Seems like the register script uses the mssql extension for php.
Since php version 5.3 the mssql extension is not available under Windows anymore.
You have to use the newer extension sqlsrv, which comes directly from Microsoft and is not part of the php 5.3 release for windows.
If your XAMPP is uptodate, it contains php version 5.3 or higher and that is the reason for "Fatal error: Call to undefined function mssql_connect()".
Here you found the new sqlsrv extension from microsoft. To use it, you might change the php.ini to load the extension and restart your webserver.
Beware, if you are not familiar with configuration of apache (or IIS) and php, you better do not change anything and try to find another register script which already use the new sqlsrv extension.
[Only registered and activated users can see links. Click Here To Register...]
:)