I don't know if I can explain this very well since you don't understand the concept and are the only user on the system. It depends on how your laptop was setup when you boot, it usually stops with a list of Users and you click your one and again depending on how you have it set-up it may just start without having to type a password.
Limited users aren't allowed to do things like save files in system folders, make changes to the system (that effect other users, doesn't matter if you're the only user), create registry entries, etc. So in a lot of cases this would mean not being able to install some programs.
Normal user is likely to have administrator privileges (but isn't the Administrator) and has the power to do many things, such as those denied to a limited user.
As I said:
try right clicking on the avast installation file, setupeng.exe and select Run As, select Administrator and enter the password
Find where you saved the setupeng.exe file, search using windows explorer and rather than double clicking on the file of icon right click and do what is quoted. You probably haven't got a clue if or what 'The Administrator' password is because you didn't set-up the system, if that is the case I really don't know how to proceed further.
Do you have the UAC pop-up on occasions, if so have you ever had to enter a password ?
If so then that is likely to be the administrator password.
