To extend a volume in Windows (up to Windows 2003) you can use the diskpart.exe utility which was introduced as part of the Windows 2003 operating system.  The following link provides information on downloading and performing the disk volume extention:
http://support.microsoft.com/kb/325590
For more information and scripting options on the diskpart.exe utility, you can review the following link:
http://support.microsoft.com/kb/300415