It sounds like DreamSeeker is open in the background.

You can open your task manager and find any references to "dreamseeker.exe" and right click them and hit "End task" or "End process", then try to reinstall BYOND.

If you're not familiar with how the task manager works, try simply restarting your computer, then installing.

If none of that works, we're going to need more details about the error messages you get. Screenshots would be great.