My Windows user account (admin) is password protected and I'm wondering if my PC can still be accessed remotely without my knowledge.
If someone got a remote access program installed without your knowledge, yes. Best thing you can do is lock down your firewall, and if you're behind a router, disable all port forwarding. That should cut down on the number of attack vectors.
Only thing you'd have to worry about then is programs that are able to traverse firewalls and routers, such as LogMeIn and CrossLoop, but they generally show up in your taskbar, Add/Remove Programs list, and/or in your task manager.