The first question is what version of Windows are you using. Different versions of Windows have different commands.
In the meantime, point at the icon you currently click on to open your internet browser. While you are point at it, RIGHT click (not left click). You should get a list of options. One should be to pin that icon to your task bar. The task bar is the list of programs that are always available at the bottom of your desktop screen.
That should re-pin the browser icon to your taskbar.
If your computer is doing strange things, I would check your antivirus software and make sure it is up to date. If you are not running antivirus software, I would get some immediately and check your entire system out. You could definitely have a virus with the way your system is behaving.
And finally, on the Windows theme, there are a variety of themes to choose from. If you chose one you don't like, you should always be able to re-choose the Windows Default. Again if you let me know which version of Windows you are running I can let you know what that version's default is called.