Don't worry about the previous question. I found the answer here: http://blogs.msdn.com/mthree/archive/2007/12/29/argentina-122907.aspx
Impact for Outlook Web Access clients in Argentina
The new time zone “(GMT -03:00) Buenos Aires” created following the recommendations on section “Updating Windows Servers and Desktop Operating Systems” will not be available on the OWA (Exchange Server 2000 and Exchange Server 2003) time zone options at this time. In order to have this new time zone available in OWA, there is a dependency on an index key which is not currently defined. We expect this to be defined over the next months.
After you configure Windows clients and servers to use the new time zone “(GMT -03:00) Buenos Aires”, OWA users using “(GMT -03:00) Buenos Aires, Georgetown” on the OWA time zone options, will receive the following warning when accessing their inbox: Use the Options page to set your local time zone. When you open a message in OWA, it will be displayed with the incorrect time during the DST period. This is related to the CDO components in Exchange and there is no workaround for this behavior at this time.
Cheers!
João Paulo H.