First things first:
You need to have gnupg or any other PGP compatible software to use this.
I will only help you with gnupg, I dont know any other... :-D

1) Save key.txt into your machine
2) Import this key to your list of keys:

[fernandoph@monalisa ~]$ gpg --import key.txt 
gpg: keyring `/home/fernandoph/.gnupg/secring.gpg' created
gpg: keyring `/home/fernandoph/.gnupg/pubring.gpg' created
gpg: /home/fernandoph/.gnupg/trustdb.gpg: trustdb created
gpg: key 722B7B2F: public key "Fernando Pablo Hauscarriaga <fernandoph@iar.unlp.edu.ar>" imported
gpg: Total number processed: 1
gpg:               imported: 1
[fernandoph@monalisa ~]$ 

3) Now, If you really trust me, just say so to gnupg...

[fernandoph@monalisa ~]$ gpg --edit-key Fernando trust
gpg (GnuPG) 1.4.6; Copyright (C) 2006 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions. See the file COPYING for details.


pub  1024D/722B7B2F  created: 2007-04-02  expires: never       usage: SC  
                     trust: unknown       validity: unknown
sub  2048g/E2AA97F3  created: 2007-04-02  expires: never       usage: E   
[ unknown] (1). Fernando Pablo Hauscarriaga <fernandoph@iar.unlp.edu.ar>

pub  1024D/722B7B2F  created: 2007-04-02  expires: never       usage: SC  
                     trust: unknown       validity: unknown
sub  2048g/E2AA97F3  created: 2007-04-02  expires: never       usage: E   
[ unknown] (1). Fernando Pablo Hauscarriaga <fernandoph@iar.unlp.edu.ar>

Please decide how far you trust this user to correctly verify other users' keys
(by looking at passports, checking fingerprints from different sources, etc.)

  1 = I don't know or won't say
  2 = I do NOT trust
  3 = I trust marginally
  4 = I trust fully
  5 = I trust ultimately
  m = back to the main menu

Your decision? 4

pub  1024D/722B7B2F  created: 2007-04-02  expires: never       usage: SC  
                     trust: full          validity: unknown
sub  2048g/E2AA97F3  created: 2007-04-02  expires: never       usage: E   
[ unknown] (1). Fernando Pablo Hauscarriaga <fernandoph@iar.unlp.edu.ar>
Please note that the shown key validity is not necessarily correct
unless you restart the program.

Command> q
[fernandoph@monalisa ~]$ 

4) That's it!, now you can verify any email message comming from me...

5) More info: man gnupg
              http://codesorcery.net/old/mutt/mutt-gnupg-howto