Is the 7 zip encryption secure?

7zip encryption is designed to protect archived files. So, as long as the tool designers did their job well, you are safe for the second case (somebody getting his hand on the encrypted file and trying to crack it).

However, such utility are not designed to protect you against your first mentioned case (someone getting access to your account data on your machine and/or you accessing the file content regularly). Indeed, someone having taken full (or even just minimal, no need to escalate privileges) access to your system will see you use this file and will also be able to capture your keystrokes while you type your password. Even worse: an attacker will actually not even have to bother with this since the file will most probably be present in clear form in your Windows Temp directory. So, for your first threat, I would definitely recommend you to use a tool designed for this usage, like KeePass which will avoid to store encrypted data in temporary files and will provide a minimum protection when typing the password.