Hi Robin, the defaulttab on your machine could have a broken uninstall entry - try using Revo Uninstaller - it's a third party software that could help your eliminate the defaulttab app from your Control Panel.

PS. make sure you remove the defaulttab browser plugins from Internet Explorer, Google Chrome and Mozilla Firefox. If you skip this step you won't be able to change your homepage and default Internet search engine (mysearchresults.com website will hijack these settings).

After following the removal guide (link in the "references" section below) your computer should be totally free of this browser hijacker.