Windows 7: Partition merging on same drive. Slightly special circumstances.

There's only 3gb on D so I concur with Ignatz that it's easiest to move that data to C, delete D then extend C which can be done safest in Disk Mgmt. Partition or Volume - Delete
Partition or Volume - Extend

To turn off paging file on D first, navigate the path that Ignatz provided (control panel/system/advanced system settings/performance settings button/advanced tab/change button),

Uncheck Automatically Managed then highlight D and check No Paging file, click Set button.



Then highlight C and check Automatically Manage box again.

Now you can delete D and extend C into the space.