I haven't used win 7 in a very, very long time, so I'm not familiar with the interface, but things haven't changed drastically over many years in uninstalling a program.
Is Avast listed in the Control Panel, if so select it (e.g. click on the entry), that should give some options, Uninstall, Change. If not do you get any options when right clicking on it ?
A screenshot of what you see, would be helpful as I can't imagine what it is you see.