When opening a DWG or DXF file you may get this error message:
The NLS files required to convert the text entities and symbol table name in your drawing code page undefined to ANSI_1252 can not be found or are not installed on your system... To install the missing NLS files, double click the Regional Settings icon in the Windows Control Panel.
This error indicates that the file contains texts encoded in a codepage which is not supported (installed) in your Windows, or - as in this case - that the source code page is "undefined".
You may want to add the missing language support to your Windows - e.g. asian languages, cyrillic (Control Panel > Languages, Regional settings), or you can just ignore the message (especially if the codepage is "undefined"). Just check the drawings texts and symbol names (block names, layer names...) for possible garbled characters, missing accents, etc.
This message may also be a result of the ACAD.VLX virus.