OK, If you right click on the avast tray icon and select About Avast it will tell you.
But for now don't worry too much about that, I was more concerned if you had downloaded a specific 'Trial Version' could also be of 60 days, they would have you fill in a payment mandate on your credit card, bank, or other payment method. I think you would remember that.
You don't say how you tried to delete this, so what I'm about to suggest you may have tried already. If you only want the avast free version, then you can use the Control Panel > Programs and Features > Avast Antivirus > Uninstall. This would also give you an option to change the the Avast Free version as an option.