I hate to break it to you, but if you're storing that much data in an INI file, perhaps the access is frequent enough to warrant the use of a set of hash-tables.

Besides that grudge, there are two areas in which this could be improved greatly:
1) Obfuscation of variable names makes life difficult. %y, %x, %d, and %b could be a little more descriptive.
2) If I were you, I'd have written it using /fopen, /fseek, and /fwrite. Much faster and more powerful.

Other than that, it looks useful and well written. :]