Well MOST computer companies create an additional (sometimes "hidden") partition/volume on your hard disk with your ONE AND ONLY COPY OF THEIR "SYSTEM RESTORE" (aka "Recovery") SOFTWARE. If you TOUCH this partition/volume your ability to RELOAD the system to its original, out-of-the-box state WILL BE GONE FOREVER.
It is NOT space that they intend for you to use, and in fact, NOTHING ELSE should be put into that partition/volume! (So, um, how do you have "only 900 mb left" on it, since it doesn't change?? ... Unless you meant that you've only got 900mb left on THE REST of the disk, and you want to use the recovery partition now, too??)
Finally, if you muck with the System Recovery stuff, and want to use it later to essentially "reinstall you system to factory condition," then the ONLY way for you to do that is to buy a copy of it from the computer manufacturer, and it will cost you AT LEAST THE PRICE OF A COMPARABLE WINDOWS LICENSE, usually MORE. (Like $200-$400!!!
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To summarize: Do NOT muck with your "D" "drive" (partition/volume) UNLESS YOU CREATED IT.