You can loop through each letter in the textbox, check if it's a certain letter, and if so, add 1 to a local variable. I'm not at my main PC, so I can't upload a capx, but it would be something like:
Local varaible "A Count" = 0
If lowercase(mid(textbox.text,loopindex,1)) = "a"
add 1 to local variable "A Count"
If you're still having difficulty, let me know and I'll upload a capx when I get a chance.