Hi everyone!I'm a new member. Get a lot of "Drawing file is not valid" messages when I try to download/open some blocks. Many is Ok, many fail. I'm using Acad-06. Any suggestions?

This message says that the DWG file is in a newer version (format) - probably DWG2007 in your case.
You can convert DWG file formats with the Autodesk DWG TrueView 2008 utility (free for download - see the Download page here).