I copy a reply that (my colleague) Khusboo posted to another user (in our ticket system):
This is a known issue with Microsoft driver, please refer to these links:
http://support.microsoft.com/kb/235889
http://support.microsoft.com/kb/884038
The issue is because of the corrupted “Schema.ini” file which is created while Defining the Text Format through “Guess”.
In this case “Column Type” is not defined for the columns. This file is created in the same folder where the text files are present.
So as a work around you can add “Column Type and length” along with the Column Names in Schema.ini file (example “Char width 255”)
Please do these changes in schema.ini and try importing through Import External Data.
I understand it is not an issue because the file is TAB-delimited, but because it is a 'fixed-width' CSV-file (some crazy thing that Microsoft invented, I believe!)