We use 2002 in our office and seem to be unable to open files from our clients saved in release 2005. We aren't in a position to ask the client to "back-save" for us.
Anything we can do?
I believe there is DWG viewer availible form Autodesk that is a free download but this only allows you to view or print not to modify. Otherwise, You need to either upgrade your ACAD version (expensive) or request the client to save/send the file in 2000 format.