In order to completely remove the application, go to Start > Control Panel > Programs > Programs and Features, select the Paint.NET application and click on Uninstall.
If it doesn't work, use Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system and download and install Pain.NET 4.0.3 which is fully compatible with Windows Vista platforms.