Remove AVG and replace it with new version?

How can I remove an old version of AVG. The computer won't let me uninstall. Can I do it manually by finding the folder and deleting it or is this dodgey? There are viruses on the computer and I can't update this version of AVG. Computer shuts down while on internet. Cannot access the registry editor either. Also computer isrunning very slow. Could I remove the old version of AVG manually and install a new version saved on another computer? Any help please.