There are two options:
By far the easiest way is simply to delete the partition completely if there is nothing on there that you want.
Next, extend the 7 partition to use all the free space from disk manager and set it as active.
Now, Windows will probably fail to boot - Insert the Windows 7 disk and on the first screen choose "Repair your computer" it will probably find the fault automatically, but if not, click Startup repair. This will rebuild the boot sector, boot menu and everything else.
When you next reboot, everything will work.
Number 2 is the safest way.
Delete almost everything on the drive apart from system files such as the boot directory.
Look in Advanced System Settings (Control Panel > System, or Windows Flag+Pause/Break)and make sure that the page file is being stored on the d drive and not the c drive (As this will take up a lot of space - unless you want it to be seporate).
Next, go in to Computer Management > Disk Management and shrink your C Drive to the smallest possible size and extend the D Drive to take up the rest of the space.
Next go to run (Windows Flag+R) and type
msconfig Go to the Boot tab and simply delete the extra option.