1. spam comments are entered and on daily basis, I have to delete these.
Give this a read.
There are some very good plugins in the repository for combating spam effectively.
2. some changed my site title, how is it possible? I changed my admin user password. but want to know all the possibilities to do so.
Except that you are running an older version of WordPres looking at your site's HTML and scanning it with SiteCheck doesn't appear to have any problems.
You may be hacked, check if there are any new users that you did not register on your WordPress installation.