If you use Windows XP or bellow you can go to Start > Control Panel > Add or Remove Programs. In the Currently installed programs box, click on the Pegasus Mail program and then click Remove.
If you have Windows Vista or above you need to go to Start > Control Panel > Programs > Programs and Features. Select the Pegasus Mail application and click on Uninstall.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system.