The truth is, with the default settings of your Windows computer, it is trivial for any program on your system to connect to the Internet. Even someone without very great hacking skills can easily steal all of your personal information without even breaking a sweat. The following program might be helpful to prevent this:
Windows Firewall – this actually probably comes with your Windows computer. Something useful one might want to do is to totally block their internet connection except for applications one trusts, such as the internet browser. This is easily done by changing the Windows Firewall setting for Outgoing Connections from “Allow (default)” to “Block” (for inbound connections the Windows Firewall should already be set by default to block them).
After that, it would be necessary to set up an outbound rule individually every time one want to allow a program to access the internet.
A good idea is also to set the default internet browser to a browser that is blocked from accessing the internet. That way, unexpected behavior from an application that may open a website on your browser will not be able to work correctly. Otherwise, any program on your computer that says “open whatever website I want to in your default browser” will actually open the website successfully, even with your firewall blocking everything but your internet browser (after all, you need to leave your internet browser unblocked, so you could access the internet).
For all you know, if an application is suddenly opening up a browser window on your system, the destination website could be a place that will install a virus or broadcast sensitive personal information to a hacker. It is better not to trust that type of behavior and to have all such attempts open up only in a browser that is not accessible to the internet and is blocked by the firewall.
That means the blocked browser must be “set as the default browser,” and the other browser you have that you trust (that has an explicit firewall rule allowing access) should be a browser that is not set as the default system browser.
Now any attempts to open the default browser (which is almost all of the cases when an external program will open a browser window) will fail, because the default browser cannot access the internet. That way, if you want to go to a website, you can decide for yourself and go to it yourself, not trust any arbitrary program on your computer to go to whatever website it wants to. It is recommended to use something like Internet Explorer for this purpose of a default browser that cannot access the internet (though actually, I would recommend disabling Internet Explorer altogether, and maybe using some other browser you don’t like other than IE even as the default “nonfunctional” browser).