Hi Toby,
You'll need to uninstall some programs and reinstall them (providing you have the original program install files) - as you reinstall, direct them to D:\, which will put them on there instead. Alternatively if you have videos/photos/personal files (in My Documents perhaps) - move them to D:\ instead.
Another way to free space is start --> cleanup --> C:\ then that should clean a lot of old chaff on C: