For some obscure reason blender has chosen to use the path d:\ as its default path for autosaving. I have no idea why (possibly it went up one dir from my desktop or my $HOME), but it is quite annoying. Is is possible to use some kind of .blenderrc file or something to change this?
You can change the temp path in Blender. Pull down the top toolbar, click on File Paths and the Temp path is bottom right, (If nothings there then it goes to the root drive). Then Cntrl + U to save default settings.