Looking at the HD on the Acer I'm using just now, it appears that D drive is completely empty, no back up or restore files, no nothing. In this case the disc is split into two equal portions, C is a 36GB partition with all the OS and program files and D is as yet completely empty.
I think, not sure mind you, but think, that what Acer had in mind was that people might use the D partition for their music and picture files etc. whilst leaving C for program and system files.
If your C drive is full, I'd be more inclined to move a good chunk of that with which you filled it over to D, then delete all the now duplicated pics, mp3s, programs etc from C once you know they're safely on D. Better to shove the stuff you've put on about than try to start shifting the meat of the machine's OS and programs around.
Oh yes, one last word of warning. I don't know that there is any OS backup on the HD. This here Acer prompted for CD images/recovery discs to be made when it was first run, so be sure you burn some off before you do anything too brave. That said, this isn't my machine, so I've not RTFM and may be completely wrong.