As magic Windows 7 disk manager, Windows 7 built in Disk Management has the ability to shrink and extend Windows 7 32 bit and 64 bit system partition. You just need to right click in Disk Management and choose "Shrink Volume" or "Extend Volume".
But there is a huge disadvantage to extend partition with Disk Management: "Extend Volume" is available unless there is continuous space behind this partition. Otherwise, you cannot extend partition. Because Windows 7 Disk Management cannot move partition to let continuous space be behind the partition which you want to extend.
You can try to download some useful partition manager tools from download websites, such as download.cnet.com, and enter " partition manager", then choose the top downloads tool.