If you get an "invalid file" error when trying to download a posted dwg file, please download and use the free drawing conversion tool by autodesk before requesting that the file sharer repost in "your" acad compatible version. I grumbled at first about this error myself, but find it is a LOT faster using this tool than it is to wait for the possible re-posting of a compatible file.
DWG TrueView™
Use this FREE* download to accurately view, plot, and publish authentic DWG and DWF files. And now, DWG TrueView incorporates all the functionality of DWG TrueConvert. This means you can translate any AutoCAD or AutoCAD-based drawing file for compatibility with AutoCAD Release 14 through AutoCAD 2009.
How it is used: Start the Trueview program, add files you need converted (as many as needed), and convert as required. once done, open (converted) files in your version of autocad.
reminder: Dynamic Blocks are not compatible with Acad versions older than 2006. Thank you.