How can I run a program not as Administrator?
I already tried
runas /user:guest notepad
But I need to provide a password and I don't know it.
p.s.: I only have the default console and PowerShell available.
You are running a Server Core edition of Windows?
Please notice: The guest account is by default disabled. You have to enable it first using this command. You also need administrator privileges to be able to do this.
If the account you're trying to run as has a password, and you don't know it, you're out of luck. But if you already have administrator access, you can either change that password or create a new user with your desired privilege level to run the command.
Unfortunately there doesn't seem to be a way to automate the password entry, and this is intentional; see this blog entry.