Password-Protected Notes

Depends on the application really. For account information, your best bet would be KeePass, which has password protection and encryption built in. There are standard fields for username, password, URL, etc. and a section for notes and comments.

Microsoft Word has password protection for documents, but is a bit of overkill if all you want is a notepad for plain text.

Best solution I've found so far is SecurePad plugin for Notepad++. Open-source and works as advertised. You can choose to encrypt the whole document or just the selected text. Once encrypted, the document or the selected text becomes unreadable until it's decrypted with the correct password.