Encrypt a file using a PGP key

[vr_sundar@comcast.net]

Hi,

I am new to using gnupg/pgp etc. I got someone's pgp key and am trying to 
encrypt a file using it. I am having problems doing this. I downloaded the 
freeware version of pgp on a Windows machine and looked at the key. It shows 
up as 

Type: DH/DSS
Size: 2048/1024
Cipher: CAST
Expires: Never

I have a  few questions regarding this:

(1) Does this public key have more than one key in it ( as in one each for DH, 
DSS and CAST )?
(2) CAST is a symmetric key system. How can a CAST key be in this public key? 
If so, anybody can decrypt what is encrypted, right???
(3) As for encryption: DSS is for signing, and CAST key is probably not there 
in this public key. What remains is DH. If DH= diffie-hellman, isn't that a 
key exchange protocol only? how can i encrypt using this key? Usually DH is 
used to generate a symmetric key for a session but in this case I don't 
establish a session. All i want to do is to encrypt the file and send it 
across.

What does this public key have actually and how do i use it? I'm told that it 
can be used for encryption. (I believe he has been able to interact with other 
people using it.)

I am pretty much lost right now. Any help on how to use this, pointers to 
understanding this better etc. are greatly appreciated. 

thanks
sundar