i need a Open Source Program for encrypting Email Attachments

Yes - Thunderbird with GnuPGP plugin - addon is actually engimail

https://addons.mozilla.org/en-us/thunderbird/addon/enigmail/

You need to have http://www.gnupg.org/ installed.

http://www.enigmail.net/download/

http://www.enigmail.net/documentation/quickstart-ch1.php#id2532629

for windows

http://www.gpg4win.org/ is an PGP email client.  

Below will allow you to signup for a free email signing certificate.  This will work with programs like outlook and is very secure by using SMIME.  It will also work with the Iphone with some tweaking.

http://www.comodo.com/home/email-security/free-email-certificate.php

does enigmail  and thunderbird work with internet explorer ???

Sorry - i misread your post, I didn't realize you wanted to do it through a browser.  How are you sending emails and attachments currently?

If we're just talking encrypting files then something like Truecrypt would work - i don't know of any plugins that would work directly with this though.

Can you describe what you want ultimately?

Fast encryption of attachments, attach it to email and send it off?

Use a service like sharefile.com, I know many lawyers that use it to transfer discovery materials (highly confidential)

with firefox, there are packages like https://addons.mozilla.org/en-US/firefox/addon/gmail-smime/

 - unaware of anything free for internet explorer. Note, you *can* use outlook and thunderbird from gmail (I am using both, right now!) and both support s/mime, and there are pgp/gpg plugins for both available as stated above.

i wanted a software which gets installed on computer and have its plug-ins installed in browsers as well so in case some body is  composing email he should get option there to encrypt attachments....thanks....

solved