Global Conflict

Ok I have this real old notebook (which happens to be the only computer which actually runs itunes without causing problems ...). I have to partitions on its harddrive (C and D) the system partition C is pretty much full causing the notebook to be really slow.

Can I transfer space from the D partition to C without deleting all data and totally killing my system?

Yes, even with windows itself
A small risk always remains
I did it quite often without harm though

The data on your D drive is being written from its start, meaning, in the middle of your physical drive (The partitions divide your drive in serial manner). To make the C driver bigger, all the data on the D drive needs to be moved. As far as i know windows (up to windows 7) is not capable of doing it.
but MiniTool Partition Wizard can. the home edition is free to use.

re-size the partitions to your liking, and hit the apply button. it will restart your computer and start moving files before windows loads. the time it takes depends on the size of your D partition (and writing speed).

thanks guys, Necro your program is totally working!!

But of course it does!
I used it to expand my own C partition as well as to format my phone's micro-SD card into ext4 filesystem and break it into partitions. Easily done.