Answer to life, the universe, and everything

Golfscript, 20

26,{.97+\{.}*}%=42`*

with new line, 21 chars (by Nabb)

26,{).[96+]*}%n+=42`*

Actually Nabb beat mine, here is original solution for with new line, 22 chars

26,{.97+\{.}*}%n+=42`*

This is simply generating source string and just comparing it against string from stdin.

Ruby 1.9, 46 42 39 characters

p (?a..?z).map{|a|a*$.+=1}*""==gets&&42

Assumes the input isn't terminated with a newline.

C program - 78 89

Edit: Do not print 42 when there are extra characters.

Assumes input does not have a trailing newline.

main(i,j){for(i=1;i<27;i++)for(j=i;j--;getchar()==96+i?0:exit(1));puts("42"+!!gets(&i));}

If the prefix does not match, the program exits. If the prefix matches but there is 1-3 or so extra characters, prints 2. Otherwise, produces undefined behavior.

This can be made one character shorter by changing exit(1) to fork(). Oh, and on an unrelated note, remember to save any open documents in case, for whatever reason, your system happens to lock up.