that's no real excuse. Software now a day are able to detect older version and uninstall the other version if it doesn't support multiple installation or upgrade installation. Like when installing ff16 over ff13, the uninstall is sleemless.
But worst, after uninstalling 13, the 15 is simply not uninstallable. The setup always detect it as installed even when selecting to remove it (which does nothing).
And the fact that avg toolbar and avg as default search and webpage are configured/installed even if not selected make it even worst and have nothing to do with the fact that wf13 is installed.
AVG has been removed according to Alex in the next release and as far as uninstalling goes, you should always use the uninstaller that came with the software if you plan on installing an incremental update (e.g. 10 > 15), no matter if you uninstall something, traces are always usually left behind on the system.