How can I create a dblink from Postgresql 8.3 to SQL Server (2000 and 2008)?
First off, you should be aware that PostgreSQL 8.3 will soon be going EOL.  You should probably use a more current version.

In fact, if you did use a more current version (9.1 or later), you could use Foreign Data Wrappers (FDW).  Someone has even posted source code for a TDS FDW, which should work for older SQL Server.

If you're somehow stuck with PostgreSQL 8.3, user DBI-Link as Craig Suggests.