Just because something hasn't been uninstalled doesn't mean it will slow down the system, what matters if it is still running in the background or not, and how much CPU and/or disk it is using. To see what's running and how much it is taxing the system, use the Task Manager. Hit Ctl-Alt-Del, select "task manager", allow access, then click the arrow that says "More Details" or something like that. It will show a whole big huge long list of apps, background processes, and windows processes, along with how much CPU usage and disk usage.
Incidentally you can kill it by selecting and clicking "end process" but doing it that way can potentially make the system unstable, and when you restart the computer, they'll likely just start back up again. Usually uninstalling should get rid of them. Most stuff you can uninstall by simply going into the control panel and add/remove programs. Beyond that, do a little googling on how to remove things from your startup through the system registry. This will prevent things from automatically launching on startup, but will not remove them. That way they are still available to you if you need them but don't run in the background. That's where a lot of software or for instance a printer driver will put stuff to automatically look for updates. Since you won't be connected to the internet, it wouldn't be able to find them anyway, so no sense in having it running in the background.