If you can't uninstall it using the classic method (Control Panel > Uninstall a program), try following the steps below:
Google Chrome and other Chromium-based browsers: type chrome://extensions in the address bar. Press the thrash can icon next to the Ask Toolbar.
Firefox: press Shift+Ctrl+A (or open Tools → Add-ons). Press Disable next to the Ask Toolbar. If you can't find the toolbar in the Extensions tab, switch to the Plugins tab and look there.
Opera 12.x and older: press Ctrl+Shift+E (or press the Opera button → Extensions → Manage extensions) to open the Extensions menu. Press Uninstall next to the Ask Toolbar.
Internet Explorer: open Tools → Manage Add-ons. Select the Ask Toolbar and press Disable.
After that, run CCleaner to make sure that there are no leftover rogue registry keys in your system and reboot the computer.