this is a common state, because we always save or install unnecessary files to the system.
so would you please clean your system firstly, remove or transfer those files you don't need them stay there any more
then check virus or malware
finally, if you still find the d drive stays red, ok, you can change the space.
two ways to change the space:
1. windows native disk resizer (windows seven only)
this is a good choice, to shrink or increase, but limited space you can change
2. 3rd party software
google with partition manager software, then you can find so much
here, i want to tell you
partition master is what i ever thought it best, works wonderfully with my pc, and also my company server