Yes if the passphrase is a bad one. No known way if it's a good one.
> Is it possible to crack a PGP file if I have a secure keyring but no
> passphrase?
You need access to the secret key to decrypt anything, so if the keyring
is secure (inaccessible), it is safe whether it has a passphrase or not.
The purpose of the passphrase is to make it difficult for an adversary
to use the secret key even if he gains physical access to the keyring
file--he must still crack the passphrase. If the passphrase is well
chosen, this is just as hard as cracking the secret key directly (that
is to say, for all practical purposes, impossible).
> The file I am trying to open was created with PGP 6.5.3 from
> Network Associates.
If you have access to the secret key, and it is not protected by a
passphrase, you should have no trouble opening the file.
--
Transpose hotmail and mxsmanic in my e-mail address to reach me directly.
To open a pgp file you need both the secret keyring and the passphrase for
the secret keyring.