From gnupg-w32-2.2.30 above and gnupg-w32-2.3.2 above, gpg cannot open gpg-agent. The error message is like:
>gpg -d password.txt.gpg gpg: encrypted with elg2048 key, ID AABE0D7867BFA077, created 2008-03-25 "Lu Wei <luweitest@gmail.com>" gpg: can't connect to the gpg-agent: Filename too long gpg: can't connect to the gpg-agent: Filename too long gpg: keydb_search failed: No agent running gpg: public key decryption failed: No secret key gpg: decryption failed: No secret key
"Filename too long" is confusing since that's not true. Test environment is Windows XP SP3.
Maybe it's another WinXP specific problem; but if it could be resolved easily, please take a look, and I am ready to help testing.