If you use 'admin' as your Wordpress user name, change it asap – and your password.
I use a plug in called Limit Login Attempts. You set the number of times someone from the same IP can attempt to login. If someone does try to login in more than that number of times, they are locked out for a period of time (that you specifiy). The plugin then sends you an email to let you know that someone attempted and was locked out.
This morning, I had 42 notifications that someone had attempted to login in using 'admin' (which is not my user name).
This is the most I've ever received in one day.
Be aware that hackers are always out there trying to get control of your blog!! Don't use 'admin' as a user name (I've also had them try the blog name as user name). Make your passwords complex and change them!