Newbie Q: decryption

[Jon Drukman]

A company I'm getting a data feed from sent me a public key and an 
encrypted file.  I want to decrypt it, but I don't know I'm doing.  My 
naive approach is not working:

$ gpg --homedir=/var/httpd/keyring --decrypt upc.xml.pgp
gpg: WARNING: using insecure memory!
gpg: please see http://www.gnupg.org/faq.html for more information
gpg: mpi too large for this implementation (40856 bits)

the public key is in the file "nf_key".  i thought i imported it but i 
don't how to tell if i did it right, or if it's even the right key for 
the file.

help!
-jsd-