UNICODE driver vs ANSI Driver

Hi:
   What is the difference between the windows ODBC PostgresSQL ANSI driver and the Unicode Driver?

When using the ANSI driver with a date type bind variable, we get a UTF-8 conversion error for some dates (Dates like
01/01/2010,01/04/2010) but not for others."ERROR: character with byte sequence 0xc2 0x9d in encoding "UTF8" has no
equivalentin encoding WIN1252" 

We do not get this error with UNICODE driver.

Any ideas what is happening?
Any issues with using the UNICODE driver instead of ANSI?

Thanks