It's just come to my notice that there is a free cad program out now that mimics autocad VERY closely. I've downloaded and installed it to test and found that It works very well. It will open and edit dwg files made with autocad 2008/09 with no errors.

The file is an 87 mb download from here.
It can also be downloaded via a torrent from the same source.
It can also be obtained from the May 2008 Pc User magazine.

Robert