Please set the com.eteks.sweethome3d.tools.temporarySubFolder System property with a subdirectory directory relative to the default one (i.e. C:\Documents and Settings\user\Application Data\eTeks\Sweet Home 3D under Windows Vista/7/8). With the -D option you used, it could give for example:
So, in the current version, if you want a completely different directory, you'll have to use ".." symbols to go to the parent directory until the root, then go to the directory of your choice (still on the same drive). Hope you can handle it.

I would prefer that the temporary files found in this directory are automatically deleted at program exit, but there's a bug in Java under Windows I don't think Oracle fixed it yet. Nevertheless, temporary files older than a week are automatically deleted at program launch.