Вставьте это (например), c:\bin\bcsvn.bat
@REM To configure this as the Subversion diff command, add these lines to
@REM c:\Documents and Settings\username\Application Data\Subversion\config:
@REM
@REM [helpers]
@REM diff-cmd = c:\bin\bcsvn.bat
@REM
@"C:\Progra~1\Beyond~1\bcomp.exe" "%6" /title1=%3 "%7" /title2=%5
@exit 0
, Вы будете, возможно, придется согласовать путь к bcomp.exe.