We are planning a conversion from Novell / btrieve to a Windows server and are researching which database to use. Our current considerations include MS SQL server, MySQL or Pervasive SQL. Our applications are internally developed using VB6, Cobol and Fortran. The program library consist of hundreds of programs that call btrieve routines to get access to the data. Although we have created the ddf's to use with VB6, we don't use the ddfs on a regular basis. I believe we can convert the data to SQL easily but my concern is to minimize the application code modifications. Is it possible to use Pervasive SQL and the btrieve API without having to rewrite the code? Any insite would be greatly appreciate.

Yes, you can use applications developed using the Btrieve API with Pervasive.SQL. PSQL also adds an ODBC (and other methods) but the core is still Btrieve.
The only caveat would be if your data files are v5.x format. PSQL v9 and later only support writing v6.x and later files. Converting the data files to a newer format is easy with either a command line or GUI Rebuild tool.