I'm workin with 2004 cad and this project i need to work on was created with 2007 cad. It won't let me copy/paste and it shows error when i try to. Does anybody know how to fix this problem or maybe change it from 2007 to 2004 version?
thank you

For that particular issue, have you tried open and save to a lower version other than 2k7? Or maybe you need to let whoever send you the files, change to a lower version.

This is probably the A2007 copy/paste problem solved by this hotfix
 

You could also download DWG TrueView 2008 and open your 2007 file with it and then save to a lower version.  That way if either the hotfix doesn't work or if you get another incompatible file from another version you will have a way to revert file to a goo version for you.
 
DWG TrueView 2008
http://usa.autodesk.com/adsk/servlet/mform?id=9078813&siteID=123112

Solution for Copy Paste Problem: (worked fine for me in Acad2000)
In Autocad2000 
-Create a new drawing 
-Insert the "problem file" as exploded block.
-Save it, and you can now Copy and Paste.
the hotfix for Autocad2007 may work but it doesn´t help you if you dont have Autocad2007.
Hope this helps anyone
Paulo