A subdomain prefixes your domain as subdomain.example.com.
If you're using SimpleScripts you need to specify a directory for it to install to, if the directory exists it's going to warn you about overwriting files because that's what it's going to do.
Unless you want a subdomain (forums.example.com) instead of a directory (example.com/forums), you should go back and remove the subdomain, then tell SimpleScripts to install to example.com/forums.
If you do want the subdomain then just tell SimpleScripts to install there.
Have you tried turning it off and on again?