Easiest way is to store your data files on D & put a shortcut to it in C:\Documents and Settings\[username]\My Documents (it's what I do).
Another way is to resize the partition. But this is not without its risks, so you need to backup your data to removable storage before proceeding. If you want to try this, let me know & I'll talk you through the steps using a free program, like Paragon Partition Manager 2005 click here. G