XP: Folder "Documents And Settings": How To Move

The partition C, where Windows XP is installed, became too small. Often XP crashes with the blue screen (BSOD). I already, successfully, moved the system folder "My Documents" to another hard drive, to give space, and changed the registry accordingly.

That was two years ago. Meanwhile, my space is again too small, somehow everything seems to grow on that partition. I need to give space again. I tried to copy the system folder "Documents and Settings" to another partition before making any registry change. But i cannot copy, XP reports an error saying that "NTUSER cannot be copied. The file is being used by another person or program". I also don't find any variable inside the registry pointing to that system folder.