Execute the following command in WinDbg directory
C: Program Files Debugging Tools for Windows (x86)> pdbcopy a.pdb b.pdb-vc6
the Old version of mspdb60.dll loaded the 6.00.8168.0
PDBCopy the requires at least version 6.20.9155.0 or one that implements PDB :: CopyToW2
Solution: install Win2003 SP1 DDK
goto x86 directory, you will find the dll.
6.20.9155.0
it here: C:WINDDK3790.1830binx86