I fix the Clear button after realizing that I didn't check for the case when the square has number zero. I have tried my program on a few Sudoku puzzles that I found on the newspapers and so far it has solved any puzzle within a few secs and not give me any trouble. So, for anyone looking for a Sudoku solver, v0.22 should work fine and with the Clear button fixed, you no longer have to restart the program if you wish to continue solving another puzzle.
In the future, I may implement the Random Puzzle(it's still a dummy button now) or replace it with Print Puzzle or both. I may also do something related to Shidoku and Rokudoku(hence the posts) although that may require significant changes to my program.
Changelog
v0.22(May 4, 2009)
1) Clear button now works correctly