If this D: partition contains a recovery partition I would be inclined not to mess with it just in case it damages your chances of doing a successful recovery at some time in the future.
But you are correct, your Windows partition needs a mininum of somewhere between 10 and 15% of spare capacity for proper operation and you need to free up some space. There are a number of things you can do.
First of all, uninstall any programs that you no longer use. Then, if you have data that you only refer to occasionally consider moving it to an external hard drive. And finally, remove any accumulation of temporary files. These can build up to use significant amounts of hard drive space - I just removed 7 Gb from somebody's laptop a week ago.
A very good tool for this is CCleaner, which you can download - for free - from www.piriform.com
When you run it, it opens in the 'Cleaner' window, and on the left hand side there are two buttons 'Windows' and 'Applications'. You choose which parts of your system you want temporary files removing from, but, as in the image reference below, under 'Applications' make sure that the last entry 'Regedit' is un-ticked, ie not selected.
One other point - just use the Cleaner section. Do NOT use the 'Registry' tool - this is severely not advised.
Edited by Chris Cosgrove, 10 November 2013 - 06:22 PM.